Hi 👋 Manja-Olive-Llama
At the bottom of the bar you have a number of different choices. At the far left is 'accounts' if you tap this you see all bills with pictures and balance. If you go to the 'fit' tab you can scroll horizontally through all your passes. What I understood is that you have at least 2, a Maestro and a Mastercard.
If you are in the passes menu and have a pass in your screen, for example the Maestro, you will immediately see an account with the picture and the balance. If you tap on this you get the overview of all your accounts. This is the menu where you can choose which account the money should be used for when you use it.
If you want to link the Mastercard to a (different) account, first select it in the horizontal passes menu and then repeat the steps as mentioned earlier. You can possibly link several passes to the same account.
I hope it succeeds now. 🙂