rain check
noun
1- N. Amer. a ticket given for later use when a sporting fixture or other outdoor event is interrupted or postponed by rain〈北美〉（体育活动因雨改期再赛时使用的）入场券。
1.1
- a coupon issued to a customer by a shop, guaranteeing that a sale item which is out of stock may be purchased by that customer at a later date at the same reduced price（商店在廉价商品售罄时发给顾客的）货到优先供应券; 优待券。
短语
take a rain check
- said when politely refusing an offer, with the implication that one may take it up at a later date（婉拒时用语）改日叨扰, 来日方长。