What if Mary had said no to God? Would God then be stuck with this sinless person walking around with nothing to do?
Mary’s choice to say yes to God was perfectly free because she has free will; however, because of his omniscience, God knew that Mary would say yes, so he wasn’t faced with the prospect you proposed. For the sake of argument, if Mary had said no, she would not have sinned by that decision. At that point, God could either have chosen another means by which to offer humanity redemption; or he could have chosen not to redeem humanity. The choice would have been his. Thankfully, we need not worry about it since Mary did indeed say yes to God.