The problem I have with the whole concept of returning fp after 6 months or a year or whatever really shows up in the situation where I'm not currently working on a building and someone comes along and adds some fp. Maybe a lot of fp. I didn't ask them to do this, it was their choice to do so. Maybe at some point in the future I'll work the building again, but when or if I work it should be up to me, not some arbitrary chosen time frame. Maybe others decide to add fp too. Their choice. If enough gets added maybe I'll go ahead and level it.
This also brings up the issue of when the timer starts, and who it affects. If it's on an individual basis, then that would really mess up positions, and open up sniping possibilities unbeknownst to the owner and cause all kinds of havoc. If it's global, then buildings in the sweet spot would suddenly become snipable.
Neither case is a good one, and this would create new problems where none existed previously.
Sounds like a lot of programming work and balancing issues for Inno, where there are none currently. I can't imagine Inno wanting to take this on, and I wouldn't trust them not to royally mess everything up if they did. The points are donated. They are gone. Live with the choices you made. Don't make your problem into everyone else's too. And I haven't even touched on the alt account banking issue or other issues I can plainly see coming if this were to happen...