Use this when the customer paid nothing yet, or only part.

Open Sales:

Period = month of sale.

Account = sale type.

Total amount = what you earned.

Leave Collected in full unticked. Type what arrived in Collected amount (0 if nothing arrived) and pick the Customer.

The unpaid rest is saved as money owed to you, with an automatic pending doc so you do not forget. When it arrives, close it from record a payment. Add a Reference and press Record it.