26 C.F.R. § 44.4401-3
The tax attaches when (a) a person engaged in the business of accepting wagers with respect to a sports event or a contest, or (b) a person who operates a wagering pool or lottery for profit, accepts a wager or contribution from a bettor. In the case of a wager on credit, the tax attaches whether or not the amount of the wager is actually collected from the bettor. However, if an amount equivalent to the amount of the wager is paid to the bettor prior to the close of the calendar month in which such wager was accepted, either because of the cancellation of the event upon which the wager was placed, or because the wager was cancelled or rescinded by mutual agreement, the wager need not be reported on the taxpayer's return for such month. Where such cancellation or rescission takes place in a month subsequent to the month in which the wager was accepted, credit or refund of the tax paid with respect to such wager may be made subject to the provisions of § 44.6419-1 .
26 C.F.R. §44.4401-3