Even if the German's stole it, and it was recovered by someone in the US forces, our government gave written permission to that person to keep it, and formed a contract as good as a bill of sale, that made it legally theirs, and made it able to be owned or sold. It also put the liability on the US government. If so, and it has changed hands, legally, then someone owes the people who claim that right of ownership, that our own government gave them. Once money and possession changed hands, the first time, it kind of made things moot, as now there's money or value to be accounted for. To me, either our own government needs to pay for it, since they declared it legal, or the Poles, if they want it back so bad, or collect its worth from our government, or really, from Germany. Somebody owes the last owner, because of legal ownership, and they are owed its worth. If it had never been sold, then things would not have been so murky, but still there was the fact that the US government said it was legal to become someones property in the first place. The only way it could not, would be if it was stolen, and somebody slipped it here, without the US governments knowledge, thus it would truly be stolen property. Our government can't take something, that they declared legal, and not pay a fair value for it.
However, there is a light at the end of the tunnel, in that the US could look to see what of ours may still be in the Poles possession, and demand it back too. I'm sure they could find something of equal value.