put on the back burner vs take a rain check

put on the back burner

take a rain check

verb
  • To request or accept a rain check (an agreement from a merchant to honor a special offer, temporarily unavailable, after the expiration date). 

  • In social interactions, to request a deferral of an invitation. To "take a rain check" is a polite way to turn down an engagement, usually with the implication that one is simply postponing it and another time might be acceptable. 

How often have the words put on the back burner and take a rain check occurred in a corpus of books? (source: Google Ngram Viewer )