I view it a bit differently than "he doesn't want to lose".
The Senators asked Blanche directly to limit the scope of the audit settlement specifically to the parties in the original suit (Trump and his family), and to already filed returns, not new ones. Blanche verbally agreed, but the Senators want it in writing.
That is what Trump is arguing about - he wants the freedom to commit tax fraud with impunity in the future. It's important enough to him that he wants to gamble on Republicans retaining the Senate after the election to preserve it.
This kind of gambit has worked for Republicans in the past. It's how they stacked the SCOTUS, after all. McConnell pulled that off because he was politically shrewd, and knew it was a good gamble to take. But let's face it, McConnell is probably never coming back to the Senate, and Trump has managed to bankrupt several casinos, so I doubt he is a good gambler.
I still think there's already been a deal made to clear the field for AOC to take over Chuck's seat in exchange for her to help clear the field for Gavin (because he is the Next One Up....).
They just all have to pretend that the deal isnt a thing, because no deal is official until it happens, and everyone wants to keep their options open in case it falls apart.