An even more tricky aspect would be if KG did formally retire, and forgo his salary. Say under that scenario, he later decided he wanted to come back. Per league rules, he is allowed to do so, provided all 30 teams sign off.
What if, however, the Celtics had already used their new found cap space to sign FA's using said cap space? Would the move be impossible due to CBA circumvention rules? Would KG's original contract be re-instated, or would the Celtics be free to re-negotiate a new, lesser value deal?