Bonjour Catherine !

The question "Tu as de la monnaie, n'est-ce pas ?" asks more for a confirmation that you have change: "You have change, right?".

To ask if someone has change, you can use:
Est-ce que tu as de la monnaie ?
Tu as de la monnaie ?   (the intonation can suffice in French)
As-tu de la monnaie ?  (a bit more formal, due to the inversion)

I hope that's helpful!
À bientôt !