rain check
rain check {n.}
1. A special free ticket to another game or show which will be given in place of one canceled because of rain.
When the drizzle turned into a heavy rain the manager announced that the baseball game would be replayed the next day. He told the crowd that they would be given rain checks for tomorrow's game as they went out through the gates.
2. {informal}
A promise to repeat an invitation at a later time.
Bob said, "I'm sorry you can't come to dinner this evening, Dave. I'll give you a rain check."
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Source: A Dictionary of American Idioms