Complex Interest calculation on excel

Need interest calculation on excel. In this case customer buys goods on several dates and pays periodically a lumpsum amount. We calculate interest invoice to invoice and it becomes a problem when customers pays in a lumpsum amount. When we calculate interest we face issue of splitting the amount in two parts(not necessarily equal) according to lumpsum payment. Currently our billing software is Tally. We want to calculate interest ledger wise in excel by exporting the ledger details from tally to excel