Legacy war chests can be donated in their entirety to the national party, donated in smaller increments to other candidates, used to set up a charitable (non-lobbying) organization (from which the retiring candidate cannot draw funds), used to retire any existing campaign debt, and/or returned to donors.
Greenwood can't give the whole amount to Fitzgerald (or whomever the new Republican nominee is). However, he can give as much as he wants to the RNC, which can then turn around and use that money in any number of contested races. He can also use it as seed money for a new lobbying career, as long as he doesn't exceed the normal individual contribution limits for each candidate.
This is another reason to challenge every seat, every cycle. Incumbents who never have to use their war chests can become sugar daddies upon retirement.