Xeon of Camelot
Major
I see some potential problems:
What if 2 people are trying to add at the same time? Players have already solved that (post first then donate). Inno would need to solve it too in their "simple code".
What if adding the required FPs would level the building? Just level and continue? Not everyone would like that. And what if the next level isn't unlocked? Just stall the entire swap thread for however long it takes to resolve?
Add a third factor for people posting at the same time, one with the most points gets it.
Only allow players to top up to one point below level, if the amount is less than the threads required amount the system would still allow you to post your GB.