I believed him, to be honest. Also, his agent wasn't purely doing it out of a misguided kindness for Terry, as they would also have banked their cut from it.
He was so passionate about CIN, it wouldn't surprise me at all if he hadn't personally given far more from his own personal income.
Plus, he may have been the only big-name celebrity to be paid, but most of the people working on the show would have been paid - including the camera and lighting operators, admin, caterers etc. etc. I'm guessing that the producer(s) would have been on quite a nice salary too, except they aren't household names.
I may be wrong, but my impression of Terry is that, whilst he obviously enjoyed the privileges and luxury lifestyle that fame brought with it, he was never obsessed with getting more and more once he had 'arrived'. I 100% cannot imagine that he was the driving force in garnering a salary for CIN night. Moreover, don't forget that the salary came from the BBC and not CIN, so if he did know about it, he might well have passed it straight on and thought "Better in CIN's pockets than the BBC's".
Finally, he was instrumental in ensuring that Jimmy Savile was never allowed to get involved with CIN, much as he and his supporters kept on pushing and clamouring to do. Yes, it would have been much better had JS been exposed from the outset, rather than it remaining an 'open secret' at the BBC; but at least Terry was determined that he would never get his filthy hands on CIN.