If something was donated...given freely..., then it belongs to the Museum, and hence to Mr Murray, who, over the years has had the care and maintenance of the Museum and all the stuff in it, at considerable cost, and, I suspect, not a lot of profit from it.
If people didn't want him to keep/sell or otherwise dispose of the items, they should have lent them, with a contract saying for how long etc etc.
To be honest, as someone who has donated more than one item to people, I think those things are his.
Mr Murray wants to retire now, and I wish him well.