The MFA in Boston has been researching the collecting histories of some of the pieces in its collection ... and has decided to return them to Nigeria (Geoff Edgers, "After tracing history, MFA returns 8 artworks to Nigeria", Boston Globe June 26, 2014. There was no legal case. There was no request from Nigeria.
In fact what we appear to be seeing is an internationally significant museum acting in a professional and ethical way.
Could the Director of SLAM be persuaded to do the right thing and return the Egyptian mummy mask to Egypt? Please?