(a) Scope. If, within the period prescribed in section 6511(a) for the filing of a claim for credit or refund, an agreement extending the period for assessment of a tax has been made in accordance with the provisions of section 6501(c)(4), the special rules provided in this section become applicable. This section shall not apply to any claim filed, or credit or refund allowed if no claim is filed, either (1) prior to the execution of an agreement extending the period in which assessment may be made, or (2) more than 6 months after the expiration of the period within which an assessment may be made pursuant to the agreement or any extension thereof.

(b) Period in which claim may be filed. Claim for credit or refund of an overpayment may be filed, or credit or refund may be allowed if no claim is filed, at any time within which an assessment may be made pursuant to an agreement, or any extension thereof, under section 6501(c)(4), and for 6 months thereafter.

(c) Limit on amount to be credited or refunded. (1) If a claim is filed within the time prescribed in paragraph (b) of this section, the amount of the credit or refund allowed or made shall not exceed the portion of the tax paid after the execution of the agreement and before the filing of the claim, plus the amount that could have been properly credited or refunded under the provisions of section 6511(b)(2) if a claim had been filed on the date of the execution of the agreement.

(2) If no claim is filed, the amount of credit or refund allowed or made within the time prescribed in paragraph (b) of this section shall not exceed the portion of the tax paid after the execution of the agreement and before the making of the credit or refund, plus the amount that could have been properly credited or refunded under the provisions of section 6511(b)(2) if a claim had been filed on the date of the execution of the agreement.

(d) Effective date of agreement. The agreement referred to in this section shall become effective when signed by the taxpayer and the district director or an assistant regional commissioner.