So the first article isn't working for money, it's working for women. That doesn't meet the definition of "mercenary". The second one implies that foreign volunteers just get room and board. That's not a mercenary either.
On the third article,
This part of your statement
"I'd argue those guys are there for the money and not for the 72 virgins"
Directly conflicts with the final quote within the article stating:
"It seems pretty safe to assume that most of these recruits aren’t in it for the money, so who knows what impact the salary cuts will have."
All that being said, the third article you cited implies that there exist foreign fighters that get paid. So even if most may not acknowledge it themselves, two things can be deducted from that article:
- Technically they meet the definition of mercenary as defined by article 47 of the Geneva conventions
- Some individuals may exist that are fighting for money
So I guess you're right. There do seem to be mercenaries in ISIS, at the very least by the technical definition. And though the article you cited implies that most aren't, it also implies that it is possible that there may be some individuals that are motivated by money. So I was wrong on that end. Well done.
Thank you for teaching me something new.