When actually initiating the payment … immediately before authenticating w/ the biometric …
Click the “>” next to “Change Payment Method”.
It will open another screen where you can toggle Apple Cash “On/Off.”
It will default to use ALL Apple Cash and then reach into your other payment method for any remainder.
If you want to use only “some” of the AppleCash balance, make 2x payment transactions:
The lower value transaction first - paid entirely from AppleCash.
Then turn “off” AppleCash and pay the rest from the other method with a 2nd transaction.