According to the FEC, there are several permissible things that a candidate can do with the money they’ve received while fundraising. They can use it for moving expenses, pay campaign staff, purchase gifts of “nominal” value to those who aren’t a member of their family, donate it to charity, move it to a state or local party committee, transfer it to a future campaign, or to a state/local candidate. Candidates can also return funds to their donors (although that may be hard to do), or give it to charity. What a candidate can’t (or should I say, "isn't" supposed to) do is use campaign funds for personal use. Although many items this does happen.
In my opinion, they should give it back to the one(s) who donated it to them. Where this isn't feasible, they should give it to a charity - NOT to another candidate who's still in the race.
Shuhak 