27 C.F.R. § 25.159 – Time of tax determination and payment; offsets.

(a) Time and payment. The tax on beer will be determined at the time of its removal for consumption or sale, and will be paid by return as provided in this part.
(b) Offsets. During any business day, the quantity of beer returned to the same brewery from which removed is to be taken as an offset against or deducted from the total quantity of beer removed for consumption or sale from that brewery on the day that the beer is returned.
(c) Offsets not allowed. An offset or deduction for returned beer will not be allowed if:
1. (1) The brewer was indemnified by insurance or otherwise in respect of the tax; or
2. (2) The brewer does not issue credit to the customer for the tax on the returned beer within 30 days of the return of the beer. If the tax is not timely credited after the offset or deduction is taken, the brewer shall make an increasing adjustment on the next tax return.
  (Sec. 201, Pub. L. 85-859, 72 Stat. 1334, as amended, 1335, as amended (26 U.S.C. 5054, 5056, 5061))