I assume you mean that you want the interest on the outstanding balance
before the payment for the period. For example, consider a 5-yr loan
of $120,000 at 12% with the first payment due in a month. When the
first payment is made, I would expect it to be $2000 in principal plus
$1200 in interest (1% of $120,000); so the total payment is $3200. The
remaining (new outstanding) balance is $118,000 (120000 - 2000). Do
you agree?

Yes, I agree. And the last payment in your example would be $2020 (the
remaining balance + intrest (1% of $2000). This is exactly what I was looking
for.
Thank you very much!