Re: Moral Dilemma: What would you do?
To me, it all comes down to: What were the expectations about this? Like, did he say, and did you agree to, something like: "I am leaving town for five months and I'd like to ask you to fill in for me while I'm gone. When I return, you agree to give the gig back to me."
If that's the arrangement, you have to honor it. If it was more like, "I'm moving away and I want to make sure my old band gets a good drummer to replace me. Do you want to do it?" then you should keep the gig if it is working out for you and the band.
If it's not that clear-cut, you need to recall as best you can.