Its a $25 bike, but if he professed expertise and made an offer that you accepted, and the money changed hands, its over. At this point it is up to you. I have screwed up and overpaid for stuff, never thought of asking for some money back later.
On the other hand, me, I'd give the guy $250 back IF HE PAID IN CASH. If it was a check, I would not. There are too many check scams out there where buyers get money back. And later, the check is found to be fradulent. One trick is they use some obscure bank not in your area. Anyone with a decent computer can print checks out, including cashiers checks.
No bicycle "expert" would ever think a Browning is worth $500, or $1300. The number of vintage bikes from that era worth that kind of money are rare.