This is actually quite important, as Stenvar is one of my main allies, and sometimes I'm forced to expel all of my followers for one reason or another. I also don't like the idea that a valued friend and ally continues to demand payment in order to join me as if we've never met each other before. Once is enough for me. As a result I'd really like to find a way of avoiding the payment part of rehiring mercs, maybe via the console or preferably by deleting that part of the NPC's programming.

Not OK to just get the money from his inventory?

I've checked and it doesn't seem to show his money.

I have no idea if there's a console command or anything, but I DO know that if you re-hire the merc within a day or maybe two (three???), they won't charge you any fee. And then there's always marriage, of course, if you consider that an option.