Snuffy, I don't shop for Steri. Is this client a monster chain gas station like Texaco or BP, or a tiny little thing like Philips 66? If the former, they should be able to verify your story by the video. I used to work for one of those monster chains and the cameras were so numerous and so precisely placed that when a customer gave me trouble about ID or checking large bills--at least once a day--I'd shrug, point to one of the many, MANY cameras, and say "I gotta. Big Brother is watching me." The cameras were strong and clear enough that on the newer, colorized bills, the faces of the Presidents were actually visible quite a bit of the time.
There is no audio on these cameras due to wiretap laws, but the motions of the clerk turning down your money should be quite clear. I'd say if Steri and their client are reasonable to work for, they'll realize this.