The regulation needed can be relatively simple I think.

By rule, NIL deals are not allowed to be tied to attending a school.

But what has happened is that the contracts have been made in a fashion where a player effectively does have to attend the school to collect (mid-week appearances required in the town a school is located in for example, but there are lots of other requirements that can't really be met unless you are attending the school a collective wants you to attend).

The regulation needed is to have a central body analyze the contracts and reject any that in any way tie NIL money to a particular school attendance.

If that gets done, a player can keep their NIL deal even if they transfer somewhere else. And if that happens, schools won't try to buy players... because they can't enforce the deal. You give them 5 million dollars and they'll still collect even if they transfer. Having a signing period for contracts that ends prior to transfer windows also will help with that. Your local car dealership will have to pay the guy knowing he might transfer, so they better be sure that they can get value from him regardless of where he's playing the next season.

You have to realize, it's ALREADY against the rules to tie any NIL deal to attending a particular school. They just need to enforce that better instead of only blocking deals that directly state that.