72 Pa. Stat. § 7247
Whenever a vendor is forbidden by law or governmental regulation to charge and collect the purchase price in advance of or at the time of delivery, the vendor shall prepay the tax as required by section 222 of this article, but in such case if the purchaser shall fail to pay to the vendor the total amount of the purchase price and the tax, and such amount is written off as uncollectible by the vendor, the vendor shall not be liable for such tax and shall be entitled to a credit or refund of such tax paid. If the purchase price is thereafter collected, in whole or in part, the amount collected shall be first applied to the payment of the entire tax portion of the bill, and shall be remitted to the department by the vendor with the first return filed after such collection. For any tax prepaid prior to the effective date of this article, credit may be claimed on any returns filed for the periods prior to the effective date of this article. Tax prepaid after the effective date of this article shall be subject to refund upon petition to the department under the provisions of section 252 of this article, filed within one hundred five days of the close of the fiscal year in which such accounts are written off.
72 P.S. § 7247