underspend

/AH1 N D ER0 S P EH0 N D/
noun
  1. 1

    An amount by which an allocated amount is not expended.

    โ€œAny underspends will revert to the general fund at the end of the year.โ€

verb
  1. 1

    To spend too little of the funds appropriated or allocated.

Source: https://en.wiktionary.org/wiki/underspend