Running sum of balance due (in new column) in sales and purchase invoices will be helpful. it will save my time manually calculate the amount of 5-10 invoices which are due.
Also in Unpaid invoices statement of customer.
What are you looking for that is not already available? Aged Receivables report and Summary both tell you the total amount amount due. Aged Payables likewise. A Customer Statement report will show you the situation for a specific customer.