I would try and think how I would feel if I was on the other end. Would I honor the deal? I believe I would, but I would be extremely appreciative of the buyer if he was also an honorable person and didn't hold me too a major loss over a simple mistake. Best case would be coming to a mutual agreement where you still get a great deal but the seller doesn't get screwed.
Alright...I’m not responding to your sappy ass post. But your avatar man...Jesus. You flared up my PTSD.
On a serious note to the OP. Be cool to the guy. We’ve all made mistakes. Maybe he’ll be appreciative and let you get it for a really good deal.