Kick him.

The stranger moans. Oh, OK, so he's not dead then.

"Take it", he begs, in an American accent. "Take it to Aveugle. I'll hold off the heavies".

You can:

Take the painting and run like hell back to your hotel.

Bend down and check that the stranger is OK. Well, other than the severely bruised groin, obviously.

Nod sagely, then prop the painting against a nearby wall and leave. Crazy Nazis, who needs 'em?

Ask, " Who's Aveugle?".