Topp! — Agreed! Duden defines it as an exclamation that confirms an agreement just made, often sealed with a handshake. The word comes from Low German legal language, where it named the handshake in a legal transaction.
Schlag ein! — Strike in! It asks the other person to take the offered hand and so confirm the deal.
Hand drauf! — Hand on it!
Ist gebongt. — It is rung up. Duden says it means that something is agreed and will be done as discussed. The word comes from the cash register.
Versprochen ist versprochen und wird auch nicht gebrochen. — Promised is promised, and it will not be broken. Children say it to each other.
In Goethe’s Faust, the wager with the devil is closed with the same word. Faust: Die Wette biet ich! I offer the wager! Mephistopheles: Topp! Faust: Und Schlag auf Schlag! And hand strikes hand!