If the adding amount always is the same then you could put that on the server, also get the player’s current cash on the server. Having a RemoteEvent which does not require any data sent from the client makes it a bit easier as you don’t really have to do sanity checks.
If you still would send for instance number data from the client, then some example conditions that you would have to in this case check on the server is for instance that the data is a number, is positive, number is not nan.