It would be helpful to have a little more information, but I'll take a crack
at it.  You say that you have two fields, next due date and maturity date,
and you want to calculate when the debtor will have paid off the loan.  It
seems like this should be the maturity date, but perhaps I'm wrong, as you
haven't defined what maturity date is.

Really, I don't see how you can do it given only those two fields.  It seems
like you would need to know the payment period (i.e. weekly, monthly,
annually), the amount per payment, and the total amount of the loan.  

Given the date that they started paying and the current date (use the Date()
function) you should be able to figure out how many payment periods have
elapsed.  Once you know how many payment periods have elapsed, multiply that
by the amount of payment to figure out how much has actually been paid.  Of
course, this assumes that the people are up to date on their payments, which
may or may not be the case.

Once you know how much has actually been paid, you will want to subtract
this number from the total amount of the loan to figure out how much is still
due.  This number should then be divided by the amount per payment period to
figure out how many payment periods are left.  Then, you will need to add the
total duration of those payment periods to the current date to find out when
they should have no payments left.  

This may or may not be what you need, but without any more information it's
the best I can give you.

-Chris