In other words, when these players accept NIL money, it shouldn't be no-strings attached money.

Just as in professional sports, your money has ties to commitments, and if you don't fulfill them, there are buyout clauses, etc.

All these deals should have ties to that, and if they break the contract (aka leave) then you have to buy out the contract.

Now it's all fair... there aren't rules forcing you to stay, but you can't just get off scott free when you feel like you want to leave as well.