let off vs make it up to

let off

verb
  • To forgive and not punish. 

  • To cause to explode or come out; to release. 

make it up to

verb
  • to redeem oneself 

  • To pay back; to return someone a previous good deed. 

How often have the words let off and make it up to occurred in a corpus of books? (source: Google Ngram Viewer )