Which argument specifically? Your sarcastic overtone make it difficult to detect an actual argument.
Your general remark that Polish art disappeared from Poland is true. Your implication that only Germany was responsible for that is wrong. The german state is generally cooperative, in contrast to the ex-soviet states, in giving back and trying to find solutions. Even from the private side is cooperation - some time ago some private owner gave back art to Poland, which rarely happens.
You did not provide a source for your claim that "German government is kind enough to let Poles bid for the art they carelessly lost".
