Yes, you shouldn't ever have that work. It shouldn't be work "FTers turned down" ever. It should be assigned, not offered, to a FTer. You shouldn't have the opportunity to accept it. Exceptions happen, that's your work, that's understandable. You averaging 40 hours a week is not, it's not an exception, that's not "unplanned" week after week.
This is so simple, I can't understand how you don't understand.