Me. Rev. Stat. tit. 36, § 3214

The tax paid on sales made on credit and reported by a licensed supplier, wholesaler or retailer pursuant to section 3209 that are found to be worthless and actually charged off may be credited upon the tax due on a subsequent return. If those accounts are subsequently collected by the licensed supplier, wholesaler or retailer, a tax must be paid upon the amounts so collected. The credit must be reported on the return for the month in which the charge-off occurred.