As Bob Biersack from the Federal Election Commission points out, most candidates dont have much left over to begin with. Campaigning is expensive, and leftover money gets used for bills and debts first, including expenses incurred while winding down an abandoned campaign or a lost political office.
Candidates do sometimes end up with surplus funds, though, particularly if theyre incumbent members of Congress who decide not to run for another term. State and local governments have their own rules, but those running for federal office including presidential candidates must abide by strict FEC guidelines when it comes to their extra campaign money. They can donate an unlimited amount to a charity or political party. They can also, within limits, make contributions directly to other candidates. A campaign committee can give up to $2000 per election to each candidate. If the committee is converted into a political action committee, the limit jumps to $5000 but to be established as a PAC, the committee would have to be in existence for six months, receive contributions from 50 donors, and make contributions to five recipients.
What candidates cant do with leftover money is use it for personal expenses. Retiring federal lawmakers used to be able to pocket extra cash and use it for cars, vacations, clothes, pet grooming, whatever but that changed in 1989 with the passage of the Ethics Reform Act.
http://www.factcheck.org/2008/02/leftover-campaign-funds/
Or, I think, he can put it towards his next Senate campaign. He doesn't have that much left, however. I don't know how many debts remain to be paid, but this could be a fairly moot question.