Cashapp can't do that. They can seize the deposited coins and refuse to credit it to your account since they are coming from a gambling platform which is against their terms of use, but they can't reject the transaction or reverse it. Of course they can send the coins back to the sending address after the transaction gets confirmed but I doubt they would do that.
I also kind of agree with user Knight Hider. A transaction with one input and one output doesn't look like coming from stake.us let alone not paying enough fee for a fast confirmation!