I don't really understand this ethical framework. What can be done with these paintings? Must they be destroyed, or anything else is "profiting off victims of Nazis"?
That doesn't make taking them from Nazis bad. It would be better if he took cash from them, but again he only gave the guy a painting, he didn't help the Nazis. And better he have the paintings then a major Nazi, even as stolen property. The absolute worst option would be the Nazis selling the pile of art for cash.