Tax Law, § 1087(b)

If an agreement of extension of time for assessment exists, the period for filing a claim for credit or refund, or for making credit or refund if no claim is filed, shall not expire prior to six months after the expiration of the extended period. The amount of credit or refund shall not exceed the portion of the tax paid after the execution of the agreement and before the date of filing the claim or the making of the credit or refund, plus the portion of tax paid within the applicable period as if the claim had been filed on the date the agreement was executed.