In reply to Adrienne's comments:

Every museum must have a de-accession policy and every potential donor
needs to know upfront that any donation taken in becomes the property
of that institution and they have the right to do what ever they deem
necessary.  Museum's do not have endless storage facilities.  Keep in
mind that museums hold collections for the public trust and can not
just randomly eliminate items in their collection.  If an item is not
needed, can not be cared for properly or does not fit the scope of a
museum's collection, they need to decline the donation.  This can be
done without alienating the individual or organization offering the
donation if handled diplomatically and professionally.  It is better
to let them know up front then find out later.