payment vs slush fund

payment

noun
  • The act of paying. 

  • An instance of that act; a sum of money paid in exchange for goods or services, or the transaction that conveys it. 

  • Antonyms: non-payment, nonpayment 

slush fund

noun
  • Money stored for illegal or dishonest purposes. 

  • Money that has not been hypothecated or dedicated to a particular purpose, or which is used for a different purpose than originally intended. 

How often have the words payment and slush fund occurred in a corpus of books? (source: Google Ngram Viewer )