only getting a fraction back would be a consolation to the fact that you had been trying to play the game with that person but something happened and they left and because them leaving is out of your control you get "some" of your fp back. but under normal circumstances you wouldn't even bother because if the person is playing eventually they would level that building. I think getting all back could be used as abuse. If someone had "friends" or multiple accounts then they could use it as a bank to save up large amounts of fp only to have them returned into ones bar and have 5000 10000 fp .... this would be unfair. so the bases of the idea is the under normal circumstances you don't want to abort, but if forced to abort (because your OCD about your list) you don't lose all you worked for. but it needs to be a small enough of a return that no one would be encouraged to use it all the time as "fun"