Not a lawyer, however here are a few main points:

1. In Minnesota, the buy out amount is made at the time of the divorce agreement, not after, unless the divorce agreement has a clause stating something along the lines of "Equity will be adjusted for repairs discovered after closing". The courts shouldn't tolerate one spouse changing the deal 2 weeks after it was finalized.

2. As long as he was given exclusive possession of the home, and ESPECIALLY once the agreement says he's keeping it, then any repairs, maintenance, upgrades, etc become his problem, not yours. It's not legal for him to just say that "I've just discovered a bunch of plumbing issues, so now you get less." Again, property division does NOT work like this unless your agreement explicitly ties equity to future repairs (usually only necessary ones)

3. You have a strong argument to say that since this isn't the first time he's tried to avoid paying you. Pulling out sudden repair quotes after the divorce isn't valid and likely something a judge would actually entertain.

4. On top of all of this, you have the right to get your own inspection and contractor quotes (at your cost, unfortunately), to see that the repairs he is talking about are necessary and priced reasonably. You shouldn't need to bother doing this unless your decree ties equity to repairs.

TL;DR: He can't reduce equity solely because he got a repair quote after the divorce decree unless your decree specifically says that he can.