Rain Cheque Meaning

(idiomatic) any postponement, especially of an offer

Example: any voucher or note issued by a store to allow a customer to get a special or sale price later if an item is out of stock
  The Sternwoods have money. All it has bought them is a rain cheque. - "The Big Sleep", by Raymond Chandler
  Your son can't make it, he says he'll take a rain cheque. - "Rocky Balboa"