The concern for the Players Association is a legitimate one.
While not exactly the same situation, the failed A-Rod trade to Boston brought up similar issues in MLB.
Sort of different. Arod was already signed by Texas and he wanted out and was willing to give back ~20-25M to fit into the Red Sox's payroll. The MLBPA has it written in their CBA that you can't reduce a player's contract without benefits being deemed equal in value. They don't want to set a precedent where owners can go around lowering the value of guaranteed contracts, it's bad for business.
Klay would be a FA, just not taking the biggest deal. I'm sure the NBAPA wouldn't like that either, but there's nothing technically against that.