It's a nine-foot tall Leger mural, which was commissioned in 1938 by Nelson Rockefeller for his new apartment. He also commissioned another mural by Matisse at the same time, which is now owned by the Museum of Fine Arts, Houston, which was the buyer of the Leger (for in the neighborhood of $6 million).
So, because the two murals are being "reunited," obviously the Leger was no longer held by MoMA in the public trust, to be accessible to present and future generations. And obviously potential future donors will not hear about this sale and ask, Why should I give this to you? What guarantee do I have that you're not going to sell this tomorrow? Just check the fine print in your Deaccession Police manual, under "R" for reuniting companion works.
One other interesting thing about the