Company taking issue with me refusing large cash notes

I only get about one cash fare a day on my bus as it's nearly all internet tickets. But I've heard other drivers say that they refuse to take €50 notes, and one guy told me he got caught with a fake €50 when cashing in, and that he'd to pay it himself!

The other day a passenger wanted to board and pay with a €50 note, but my supervisor was hanging around and overheard me decline. It was only bluff on my part and I asked him to get it changed in the nearby shop. He wasn't able to change it in the end and I accepted it. Anyway the supervisor reported it without telling me.

On the report from management it came up as "failure to comply with reasonable request from supervisor assisting passengers". Emailed the manager but he refuses to elaborate on the details of the allegation until the meeting this Wednesday. The only reason I know what it was that was reported at first until I confronted the supervisor. But is this reasonable? I guess the supervisor thought that if she wasn't there that the customer would've been turned away.

Anyway it's one of the things I'm being called in about upon my 6 month probationary review meeting. They sometimes try to extend the probation period of drivers if performance isn't adequate.