I got busted with 31.7g of Marijuana, 4 pipes, and a scale in Oregon.?

I got busted on May 20th, for having 31.7g of Marijuana, 4 pipes, and a scale. This is my first time offense in anything. I am 18 years old. I have a full time job. I told the officer that the scale was to verify what I was buying for personal consumption, I don't trust anyone elses scale but mine. I don't know if that helped at all or not. But he wanted to cut a deal with me. He told me if I could get him a bust bigger than marijuana, he would let me go. I told him okay, so I am leading him to a guy I know does heroine, cocaine, and sells shrooms. He wants me to go in and buy a pound of mushrooms from him while wearing a wire. and if this leads to his arrest, he will make it so nothing ever happened with me. First off, if he let me go the night of with just an evidence report, can he technically still arrest me? Second off, is this legal what he is doing? Third off, if things go south, what charges could I be looking at? Anything helps! if you have any questions, let me know and I will put the answers in the additional details!