Interest on a Buy Here Pay Here deal is calculated from the current principal balance. This will be the amount financed at the beginning of the deal. The interest will start accumulating the day after the sale date.
The Amortization Schedule in the Payments tab will show how the interest and principal will breakdown if your customer pays the exact amount on the exact dates defined in their contract.
The daily interest is calculated with the following formula:
Using the example above we see the amount financed was $10,000.00 and the interest rate was 7.5%. If no payments were made in one year $750.00 in interest would accumulate.
Dividing this by 365 shows about $2.05 of interest accumulating per day.