Summary
Sometimes you have a customer pay for more than one service when you're out in the field using the mobile app. When you make the payment you don't have the option to apply the extra money to an existing unpaid invoice.
Key Points
This feature is not available in the original Fieldwork mobile app
Use the Fieldwork Office app to pay an entire balance for a customer
This is by design as most businesses have at least one non-owner tech in the field and the owners don't want payment manipulation available to these techs.
If you want to apply the extra payment at that time, Complete & Save the work on the mobile app.
Open your device's browser and go to https://app.fieldworkhq.com/
Open the customer's account and apply the payment there.
Use the Fieldwork Office app to pay an entire balance for a customer. For more information see: Fieldwork Office - Getting Started Guide