Mannlicher,
I know he's an adult, all's fair, etc, but if a friend or associate (especially a subordinate) offered to sell me his grandfather's M1 Carbine for about 1/20th its market value, I don't think I could live with myself if I took him up on it without at least informing him that he could almost certainly get much more elsewhere (maybe you did so). I can't imagine trying to talk him down another $10 - much less bragging about doing so. I assume this is someone you are going to see daily. I'm sure it'll do wonders for your working relationship (and his trust in you) when he finds out how you essentially stole it from him. I like finding (or negotiating) a bargain, but taking such extreme advantage of someone's ignorance (especially someone who knows and trusts you) seems like a little too much to me.
Doug