Hi,

By default in Odoo invoicing if you pay 100 usd against 70 usd, the extra 30 is stored as extra amount/credit of the partner.

If you want to show the amount payed and returned amount, either you can alter the flow of the keeping the balance as credit. it will be little tough for you. Simply what you can do is just add two new fields in the invoice model to store the paid amount and the returned amount. Once you done it, you can inherit the report and add those fields to it.

For adding new fields to existing view, you can have a look at this blog: Add Custom Fields to Existing Views

For customizing existing report, check this: Inheriting and modifying QWeb report

If  you feel above methods are tough, you can log a note in the chatter section below the form view to record the given amount and returned amount. But this will not still show in the report. In the invoicing you can see a field named terms and condition, if you are not using or supplying a terms and condition in it, you can just enter the paid and returned amount in it, it will get displayed in the report.

Thanks