RAINCHECK

Any voucher or note from a merchant to a customer to provide an item the merchant has run out of at a later date for the item's current price, or (in a more literal sense) to provide a service at a later date.

In social interactions, a polite way to turn down an invitation, with the implication one is simply postponing it and that another time would be acceptable.

Example: "I can't go with you to the museum this saturday, but can i take a rain check and go some other day?"