I couldn't find a forum that seemed exactly right for this topic so if I'm posting in the wrong place I'm sorry.
I sold my old Yamaha bass half an hour ago to this guy and just realized I forgot to give him the key to the case! I can't find it, either

it's been over 2 years since I really even played that bass so I have no idea where the key is.
I also forgot to give him the manual, so he's coming back this afternoon, and he did ask about the key as well. Crap.
What do I do? Give him back some of the money? How much? It shouldn't be that hard to have the case re-keyed, right? I did say in the ad it was the bass itself for sale and I was throwing in the case for free...and I let him pay like $75 less than what it should have gone for and gave him the $125 case for free (minus the key of course, *facepalm*).
I'm thinking of just putting the manual in an envelope and taping it to the door with part of the money and an apology note and going over to a friend's place to hide. I have a key from an old lockbox that looks sort of like it could be for the case that I could leave and play dumb

I'd feel really bad, that's really crappy but it was an honest mistake...

What do you guys think I should do?