You don't mention which version of Crystal you're using, but I'm going to assume that it's fairly current.

This is fairly easy to do. Crystal has a "Running Total" option that you would use. Basically, you would set it up like this:

Field to summarize: Payment field
Evaluate: For each record
Reset: On change of field Invoice Number field.

For my example, I will call this {#Payments}. The other assumption I'm going to make is that the first row that you're displaying comes from a "master" record that has the information about the total amount of the invoice. Then you would create a formula that looks something like this to display the running balance:

{Invoice Balance} - {#Payments}

Depending on how the data is set up, if the payments are already negative, you would use "+" in the formula instead of "-".

-Dell