Doesn't this end up in some other poor sap then? Wouldn't you want to have the blood collected and destroyed as medical hazmat, along with testing to confirm burden reduction?
So the question is whether they need the blood more than you did pre-transfusion. If they did, then you're still commiting a net positive. If not, then it serves them right for requesting a blood donation despite needing it less than did an individual who was ready to donate their own.
Unless the blood is going to research, in which case it is a net positive regardless.