Until there is a system of royalties for living artists with each sale of artwork, like there is for music, artists have to close their eyes to what happens to their work after the first  sale/donation.  

comeing from an anonymous donor.  That happens quite frequently and for a variety of reasons the one you note among them.  As far as it being an insult to the artist... you bought their piece so it's yours.  I'm sure they'd love to see it fetch an even higher price than you paid.  

I've had one bad experience with an artist.  He objected to the base on which a sculpture of his given to us by a donor, stands.  It was black granite not black marble but the donor who purchased it was not sold the black marble base by the artist, just the sculpture.  The donor hadn't uncrated it since purchased some 30 years before.  The artist is of some reknown and so was able to make a bit of stink but it's still on a black granite base.  The sculpture is intended to be installed out of doors.  Black marble would have been ridiculously expensive and would not have weathered well.