I also may have mis represented what it was, It was a bucket of greenish powder
Whatever it was, it wasn't brass, in any shape, manner, or form. Putting it in the brass barrel would be like putting table salt in the Sodium Metal barrel. Brass, as defined, is a metal alloy and copper oxide is not even a metal. Like table salt, copper oxide contains metal, but it isn't a metal - huge difference. As a scrap item, especially in small quantities, the value of that copper oxide is worth far less than brass, no matter what percent of copper it contains. Actually, it is a liability, since the EPA would consider it hazardous heavy metal waste. I hope that drum of brass doesn't get rejected. If I were the buyer, I would surely reject it if I caught it. If I didn't catch it when I paid you, but caught it a couple of days later, everything you brought in in the future would be suspect, at least until I started trusting you again. Every drum you brought in, for awhile, would have to be dumped out and examined.
Honestly, I'm not trying to be a jerk, although I probably sound like one. I just think you are making a mistake and it will come back to haunt you. You might get away with it once but, it you continue doing it, it will come down on you. I think you took the ISRI listings a little too literally and read your own private interpretation into them.
You might call your buyer and ask what he pays for copper oxide. Actually, according to the CRC handbook, pure copper oxides are either red or black. It sounds more like a mixture of hydrated hydroxides and sub-oxides to me. Also, I have no idea how accurate that x-ray gun is on non-metallics, but I would sure question it.