rain check

1. A ticket stub entitling the holder to admission to a future event if the scheduled event is canceled because of rain.
2. An assurance to a customer that an item on sale that is sold out or out of stock may be purchased later at the sale price.
3. A promise that an unaccepted offer will be renewed in the future: declined the invitation to dinner, but asked for a rain check.

A promise or commitment by a seller to a buyer that an item currently out of stock can be purchased at a later date for today's sale price.

Investopedia Says:

The term originated from baseball; spectators at games that were postponed because of rain would receive a check that could be used to attend a future game.

A promise that an unaccepted offer will be renewed in the future, as in I can't come to dinner Tuesday but hope you'll give me a rain check. This term comes from baseball, where in the 1880s it became the practice to offer paying spectators a rain check entitling them to future admission for a game that was postponed or ended early owing to bad weather. By the early 1900s the term was transferred to tickets for other kinds of entertainment, and later to a coupon entitling a customer to buy, at a later date and at the same price, a sale item temporarily out of stock.

The noun has 2 meanings:

Meaning #1: a promise that an unaccepted offer will be renewed in the future

Meaning #2: a ticket stub entitling the holder to admission to a future event if the scheduled event was cancelled due to rain