I had a very similar experience not long ago....
My father offered me a bunch of old guns from his locker. He told me they were "about worthless" and that I could do what I want with them.... with the exception of two guns which were handed down from my grandmother and grandfather respectively, which I was not to sell (no problem)....
Well, I get the guns home and go to check on the value of these "about worthless guns".... first grandmas- one of the most produced single shot .22's in the world... about $100 (yup, he's right).... next grandpas- it would seem this gun is second only to grandmas .22... again, about $100.... next gun, a nice .308 that had sentimental value to me... about $250... the next gun which had no family value aside from someone left it at my grandpas house many many many years ago, on checking and rechecking values everywhere I can, is a clapped out old WRF 22 (caliber is out of production for a long time)... kind of rusty and scared.... and is worth more than any other gun in my collection.... and more than most of them combined!!
I had to give that some hard thought, but then I called my dad and told him what I had found out.... he said "thats fine, do whatever you want with it".... he being my dad I told him I didn't feel right about selling it from under him, if thats what I decided to do with it... he said "fine, I tell you what, you keep it, and if you ever sell it, give me half".... sounded like a good deal to everyone....
I feel good, I have this true collectors firearm in my collection, and I know I can do whatever I want with it without any guilt....
I'm not a big sucker for sentiment, so it might go someday. I would never put a painting this valuable on my wall, and it's useless to shoot (if I could find ammo). It's not all that good looking aside from being obviously old and rustic. And I wouldn't trust that it wouldn't get stolen if on display in my neighborhood.... we'll see...
The moral is.... tell him what you really know, including that you really want this gun and that this would be an overpayment that you can (or cannot) pay back.... tell him you want him to know that, if he were to go through all the trouble, he could likely sell it for more.... odds are he will tell you to take it as opposed to deal with the issues, and knowing it is something you really wanted.... perceived fate is hard to argue with....