Let a 50 cent stamp hold up a $650 sale? Tear up a check without your say so instead of sending it back? He's not making himself look good.
I think there is a way to cancel a Cashier's Check, but it takes time and costs more than a little.
With a phone call, a Deputy Sheriff will knock on his door. I know because it happened to me. I sold a part to Bosnia. Told the guy there would be no shipment until the check cleared. He got impatient and called the local PD. An officer knocked on my door to ask what was going on. He couldn't tell me to ship it, but I did have a cop car in front of my house and an officer on my porch. If I was a scammer, it would have given me pause.
In the end, there is small claims court. It can be conducted over the phone on his end, you would go to your local court house.