ONEOK (OKE)

ADJUSTMENT OF PURCHASE PRICE

(a) At least three (3) days prior to the Closing Date, Seller shall prepare and deliver to Purchaser a statement (the “Estimated Closing Statement”) setting forth Seller’s good faith estimate of the adjustment to the Purchase Price (the “Estimated Purchase Price Adjustment”), which shall include a reasonably detailed calculation of the amount of all capital contributions (or other payments required to be made by the Enron Sellers in their capacity as equity holder) made (or are expected to be made on or before the Closing Date) by the Enron Sellers or their Affiliates (other than a Transfer Group Company or Northern Border Company) directly or indirectly to a Northern Plains Group Company or Northern Border Company (the “Northern Capital Contribution”) for the period beginning with the first day after the Balance Sheet Date and ending on the Closing Date (the “Northern Capital Contribution Amount”). The Estimated Purchase Price Adjustment, shall equal the Northern Capital Contribution Amount. The Estimated Closing Statement prepared by Seller shall be based upon the Estimated Closing Statement (as defined in the Enron Purchase Agreement) to be delivered to Seller pursuant to the Enron Purchase Agreement.

(b) Purchaser shall prepare and deliver to Seller, within twenty (20) days following the Closing Date, a statement (the “Final Closing Statement”), which shall include a reasonably detailed worksheet setting forth the information specified in subsections (i) to (iii) below. Seller shall grant Purchaser and its Affiliates and Representatives access at reasonable times and places to all books, records and employees of Seller (in each case, only those portions or aspects as they relate solely to the Transfer Group Companies) that is reasonably requested by Purchaser in connection with Purchaser’s preparation of the Final Closing Statement or in responding to any Objection.

(i) a reasonably detailed calculation of the Northern Capital Contribution for the period beginning with the first day after the Balance Sheet Date and ending on the Closing Date (the “Final Northern Capital Contribution Amount”);

(ii) a calculation of the aggregate purchase price adjustment (the “Final Purchase Price Adjustment”), which shall equal the Final Northern Capital Contribution Amount; and

(iii) a calculation of the true-up amount (the “True-up Amount”), which may be positive or negative and shall be the result of (A) the Estimated Purchase Price Adjustment minus (B) the Final Purchase Price Adjustment.

(c) Seller shall have forty-five (45) days from its receipt of the Final Closing Statement (the “Objection Period”) to review the Final Closing Statement. Upon the expiration of such forty-five (45) day period, Seller shall be deemed to have accepted, and shall be bound by, the Final Closing Statement and the calculation therein of the

Final Purchase Price Adjustment, unless Seller has informed Purchaser in writing of its disagreement with the Final Closing Statement prior to the expiration of such forty-five (45) day period (the “Objection”), specifying each of the disputed items and setting forth in reasonable detail the basis for each such dispute (each, a “Disputed Item”). Purchaser shall have twenty (20) days from the date on which they received the Objection (the date on which such twenty (20) day period ends, the “Objection Date”) to review and respond to such Objection. If Purchaser and Seller are able to negotiate a mutually agreeable resolution of each Disputed Item, and each signs a certificate to that effect, the Final Closing Statement and the calculation therein of the Final Purchase Price Adjustment, and, if applicable, the True-up Amount, as adjusted to reflect such resolution, shall be deemed final, non-appealable and binding for purposes of this Agreement. Purchaser shall grant Seller and their Affiliates and Representatives access at reasonable times and places to all books, records and employees of the Northern Plains Group Companies, and shall use commercially reasonable efforts to provide such access with regards to the Northern Border Companies, that is reasonably requested by Seller in connection with Seller’s review of the Final Closing Statement.

(d) If within thirty (30) days of the Objection Date any Disputed Items have not been resolved in accordance with paragraph (c), Seller and Purchaser shall refer such Disputed Items to an accounting expert (the “Accounting Referee”), within five (5) days after acceptance of appointment by the Accounting Referee, to make a final, non-appealable and binding determination as to such remaining Disputed Items pursuant to the terms hereof. The Accounting Referee shall be selected by mutual agreement of Purchaser and Seller, but in the event that an Accounting Referee has been appointed under the Enron Purchase Agreement to resolve a bona fide dispute thereunder, such Accounting Referee shall serve as the Accounting Referee hereunder; provided in the event that no Accounting Referee is appointed pursuant to the preceding provision within fifty (50) days of the Objection Date, Seller and Purchaser shall each, within sixty (60) days of the Objection Date, select an accountant who is a partner at a nationally recognized firm of independent public accountants, who shall be directed to select, within seventy (70) days of the Objection Date, a third accountant to serve as the Accounting Referee; provided further that any Accounting Referee appointed pursuant to this sentence shall be an active or recently retired certified public accountant or accounting expert with substantial experience with assets and complex financial transactions of the type set forth in the Agreement. The Accounting Referee shall be directed to make a determination in accordance with paragraph (e) below of the Disputed Items promptly, but no later than sixty (60) days, after acceptance of its appointment. Seller and Purchaser agree to use their commercially reasonable efforts to effect the selection and appointment of the Accounting Referee pursuant to this paragraph (d), including, without limitation, executing an engagement agreement with the Accounting Referee providing for reasonable and customary compensation and other terms of such engagement. Seller and Purchaser shall make readily available to the Accounting Referee all relevant books, records and employees of the Northern Plains Group Companies, if applicable, and shall use commercially reasonable efforts to make readily available to the Accounting Referee all relevant books, records and employees of Northern Border Companies, if applicable, that are reasonably requested by the Accounting Referee in connection with the Accounting Referee’s review of any Disputed Items; provided that Seller, Purchaser and

their respective Affiliates shall not be obligated to provide any information the disclosure of which would jeopardize any privilege available to such Person relating to such information or which would cause such Person to breach a confidentiality obligation to which it is bound; and provided further that Seller, Purchaser and their respective Affiliates shall use their best efforts to minimize the effects of any such limitations.

(e) If Disputed Items are referred to the Accounting Referee for resolution pursuant to paragraph (d) above, the Accounting Referee (i) shall determine only with respect to the Disputed Items submitted whether and to what extent, if any, the Final Purchase Price Adjustment set forth in the Final Closing Statement and, if applicable, the True-up Amount requires adjustment, and (ii) shall not assign a value to any item greater than the greatest value for such item claimed by either party or less than the smallest value for such item claimed by either party. Any finding by the Accounting Referee shall be a reasoned award stating in reasonable detail the findings of fact (if any) on which it is based, shall be final, non-appealable and binding upon the parties and shall be the sole and exclusive remedy between the parties regarding the Disputed Items so presented. The fees and expenses of the Accounting Referee shall be borne by Seller and Purchaser in the same proportion that the dollar amount of Disputed Items which are not resolved in favor of Seller or Purchaser (as applicable) bears to the total dollar amount of Disputed Items resolved by the Accounting Referee. For illustration purposes only, (A) if the total amount of Disputed Items by Seller is $1,000, and Seller is awarded $500 by the Accounting Referee, Seller and Purchaser shall bear the Accounting Referee’s fees and expenses equally; or (B) if the total amount of Disputed Items by Seller is $1,000, and Seller are awarded $250 by the Accounting Referee, Seller shall bear seventy five percent (75%) and Purchaser shall bear twenty five percent (25%) of the Accounting Referee’s fees and expenses. Seller and Purchaser shall bear the fees, costs and expenses of its own accountants and all of its other expenses incurred in connection with matters contemplated by this Schedule 2.1.

(f) If the True-up Amount is (i) a positive number, then Seller shall pay Purchaser such amount or (ii) a negative number, then Purchaser shall pay Seller such amount. Payment of the True-up Amount calculated pursuant to this Schedule 2.1 shall be made (i) if no Objection is made by Seller during the Objection Period, within ten (10) days following the expiration of the Objection Period or (ii) if Seller submits an Objection within the Objection Period, within ten (10) days following final resolution of all Disputed Items by the parties or the Accounting Referee, by wire transfer of immediately available funds to an account designated by the parties receiving such funds, plus interest thereon from and including the Closing Date through and including the day before the date of such payment, at a per annum rate equal to the Prime Rate as at the Closing Date.