As of right now, he is already using those funds for registration purposes. He can make a hell of alot more money, if he can create a sizable group of loyal dem voters, and if he can have the kind of credibility with the democratic party that he wants. Pocketing the money at this point and time would be short sighted.
As a side note, his goal of registering 2 million new voters and getting a "get out and vote" network, seem to be making progress, especially in south carolina (yep, he's been working down there) and north carolina.