My suggestion is simple. Implement a feature that is widely used in the banking industry. For gold transactions, there is a "pending" period. In this time, you own the gold but can not use it until the pending period expires. I would say 1 hour would be plenty(banks must wait for proof of a valid transaction before they can officially process it. This way anyone planning on scamming then transferring the gold to a multi will not be able to. Nor will they be able to dump it by purchasing other items.
This only works if the mediators tend to the accusations in a timely manner. On thing that would also help would be for the mediator to be able to put an immediate lock on the funds in question(not other funds, only the funds being questioned). This again is common in real life civil suits where peoples assets are frozen pending the outcome. It does no stop them from doing whatever they want with any funds that aren't in question, but prevents them from unloading/hiding any that are.