Re: Strange little professional problem
NO WAY should you have even been ASKED to chip in for rehearsals as a sub.
It's his gig, and you are being paid to do his show as a sub, not a member of the band.
You are the one providing the service. YOU could do another job, but he couldn't do this show without you doing this for him.
I'd set him straight (nicely of course), and if he insists you pay, I'd tell him to find someone else. You could also let other drummers around town know about his "business practices" if he gets snippy about it.
You don't want jobs that are going to cost you money.
...Of course, it could just have been a "I didn't think of that, sorry" too, so maybe tread lightly before bringing out the big guns :-)
Good luck with it!