The payback method is always briefed when the fri is given. So know that you will have to pay it all back shouldn't be a surprise if you renege/leave etc.
But that is missing the point. If someone welshes on a ROS linked to an FRI then they will pay the penalty. If people applied for higher rate flying pay and were properly processed and approved by admin staff, then that is no fault of the recipient. Sack the chief blunty for missing such a gross error! I think a good lawyer should be able to fight the overpayment charge, MOD legal is as much use as a chocolate teapot