Clawback
Grant money comes with conditions. Where those conditions are not met — the project changed, the money was not spent, the outcomes were not delivered — most agreements let the funder ask for it back.
Clawback provisions typically bite on unspent funds at the end of the grant period, on money spent outside the funded purpose, and on failure to deliver agreed outputs. Some agreements also allow recovery where the charity failed to report.
The practical rule: talk to the funder before the year end, not after. Most will agree a variation or an extension if asked in time. See handling an underspend.