Also, I am doing a test to see if I can figure it out. I created a sales order with 3 items. 1st installment, 2nd installment and 3rd installment. Converted the order to an invoice and invoiced only the first installment. Then I applied a payment to that invoice. Then I went to open the sales order and was hoping that it would show the 1st installment paid. Instead it shows the full amount as owing. Can anyone shed some light on this please?

go to this link to see how to do a "progress billing"  https://help-sage50.na.sage.com/en-ca/core/2018/Content/Projects/ProgressiveInvoice.htm

this will help you out!