I'm a cashier at a retail store and wanted to do an exchange for some product that I bought. I processed the exchange myself because it's much quicker since the system requires my personal information. I didn't steal any money or product. The next day, my manager confronts me about it and says he can terminate me right then for putting through my own transaction because doing so is against company policy. This was on Sunday the 16th, I was scheduled to work the next Saturday and Sunday on the 22nd and 23rd respectively but he cancelled them while they investigated. He told me I would be informed of their decision the week after. On the 26th, my manager called me and told me the decision came from above him and said they’ve decided to terminate my employment. I worked there for 8 months and didn't do anything wrong up until that point. It was the first mistake I made and I didn’t receive any warnings. He said they didn't find anything wrong in their investigation but the fact that I put in my own transaction was enough because they feel I can’t be trusted at the cash even though I’ve done nothing wrong. Have I been wrongfully dismissed or owed any compensation?

Is there a valid cause for terminations in this case? I haven't been informed of any termination pay I'd be receiving but it has only been two days.