Roman Rewald, a Warsaw-based lawyer who has represented the U.S. Holocaust museum in the past on a pro-bono basis and has knowledge of the current discussions, says the matter comes down to Polish law, which is rigid and hard to work around.
The law would do a good job, for instance, of stopping an official from giving away a precious 16th century painting, but isn't as well-suited to regulating Holocaust artifacts, which probably shouldn't be moved so often.
"The Polish law is designed to make sure that nobody has any leeway in allowing Polish artifacts to leave the country permanently," Rewald said. "Poland is trying to protect its artifacts, all of them. Unfortunately Holocaust artifacts, which Poland has an abundance of, fall into the same category as all the other artifacts which Poland has been robbed of during wars, especially World War II."