raincheck

Concise Oxford English Dictionary © 2008 Oxford University Press:
rain check

▶noun N. Amer.

• 1 a ticket given for later use when an outdoor event is interrupted or postponed by rain.

• 2 a coupon issued by a shop, guaranteeing that a sale item which is out of stock may be purchased at a later date at the same reduced price.

– phrases
take a rain check refuse an offer, with the implication that one may take it up at a later date.