Service Invoices for appropriate days credit values can be created for the appropriate target dates.
A Quick Entry for corresponding credit as discount for the actual invoice to show the current payment can be passed as well.
Main Order / Invoice Items Portion:
Today:
Sales Cr 1000
Client Dr 300 (30% on order / invoice)
Dummy Discount Dr 700
On first payment:
Client Cr 300
Cash/Bank 300
On Expected Date:
Dummy Discount Cr 300
Client Dr 300 (30% on delivery)
On second payment:
Client Cr 300
Cash/Bank 300
90 days after delivery:
Dummy Discount Cr 400
Client Dr 400 (balance 40% settlement)
On final payment:
Client Cr 400
Cash/Bank 400
This way at anytime, the Dummy Discount Account will always show what needs to be got from the client after a later date.