You're never concentrating the PFAS, so at worst you're bringing poor sap's new PFAS concentration closer to your own pre-transfusion levels.

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.