This seems to happen so much through history and it seems that there is always a loser in the situation. Museums are filled with foreign art through the conquest of war. I have often asked myself the question if I owned a piece of art that was handed down through generations and found out it had been acquired fraudulently how would I react to being the person to decide to give it back or keep handing it down through the generations? I would hope I would be the one to correct the offense and give it back.
It does seem that there could be agreements made as either purchasing the art legally, giving it back or agreeing to be co-owners letting the art tour through out the countries and the money split between the two.