Repayment of an interest-bearing loan with heiter iska

Reuven lent a sum of money to Shimon and they signed a heiter iska (a document that turns a loan into an investment, thus allowing to lender to charge an interest). Shimon missed some of his monthly payments, which he is supposed to make according to his agreement with Reuven and according to the heiter iska. Can Reuven now add those missed payments to the principal, and receive his returns from them as well, according to the terms of the heiter iska? Or does he have to consider those missed payments as a separate loan? (We’re talking about a case where this was not stipulated beforehand.) Aforementioned Shimon returned part of the principal. Can Reuven now subtract those missed payments from the sum that was repaid, and count the rest of the money as repayment of the principal? Or does he have to subtract the whole sum from the principal, and the missed payments will be considered a loan (either as part of the principal for the purpose of calculating the profit, or not, depending on the answer to the first question)?

Dear …!

The answers to both these questions depend on the exact text of the heiter iska that you signed.

Most of these documents a written in a way that will not allow you to add missed payments to the principal of your loan.

However, if you use the heiter iska that was drawn up by our teacher Rabbi Amram Fried, missed payments can be added to the principal, and one is allowed to charge interest on them.