Do Unto Others
Dude, it's worth hundreds, not A hundred -- if he's really your best friend I'd a) I'd tell him now you'll give it back to him when you're done renting it until he can afford to buy shells again b) sell it and split the profit with him c) pay him the rest of what it's really worth now*.
*I'd get a quote from a coupla gun shops and pay him the difference of what they offer you on average -- that way you don't rob him and he doesn't lose out, will continue to trust your friendship and be happy it found a good home with his pal, and you STILL got a great gun at dealer COST not to mention being able to look yourself in the mirror and sleep at night.
Tough love? Sorry. Step up brother!