Giving the OP the benefit of the doubt, I interpret their question as is it safe to reuse the returned points from the cancelled reservation for another stay? If not, the next award reservation they want to make will potentially put them at a deficit in their MR Bonvoy account when the points for the previous Austria stay are deducted. By the way the OP worded the question, I think they already know the answer. It isn't safe to do so. As others have already pointed out, Marriott will eventually figure out the mistake and deduct the points for the Austria stay. So the points for the stay in Austria should be considered encumbered. Perhaps if six months or a year passes and the points still haven't been deducted at that time, it might be safe to assume the hotel will never figure out their mistake and reuse them for something else. YMMV...

--Jon