Creating a master-detail relationship on the Payment with a Roll-up Summary field on the Invoice will build this functionality. A master-detail relationship creates a parent-child relationship between two objects, where the child records inherit the sharing and security settings of their parent. A Roll-up Summary field calculates values from related child records, such as count, sum, min, or max. In this case, the Roll-up Summary field on the Invoice can calculate the sum of all Payments related to it and subtract it from the Invoice amount to get the current outstanding value. References: https://help.salesforce.com/s/articleView?id=sf.
relationships_considerations.htm&type=5 https://help.salesforce.com/s/articleView?id=sf.
fields_about_roll_up_summary_fields.htm&type=5