The DNC can return it to donors if they request it back. The DNC can also use it for future elections.
A smart move by Obama and worth much more than he can buy ad time with is to go out and say that he will use X amount of the donations for some charity program (unless its against the election laws).
But he wants the seat way more than he wants to help
And you can't say that he will shot himself in the leg since he got PLENTY of extra cash for the election.