To calculate the balance of a fixed rate loan and its cumulative interest paid, use the MORT function. The following example illustrates this approach:

A loan has an initial amount of $100,000 on 01DEC1989, and a payment of $725.07 is made on a monthly basis beginning 01JAN1990. Interest accrues at an annual rate of 0.07875 that is compounded monthly. The remaining balance is paid off in a single balloon payment that is made immediately after the 01DEC2000 payment (that is, after 10 years and a total of 120 monthly payments). What is the amount of the balloon payment?

data one;
   amount=100000;
   payment=725.07;
   rate=0.07875;
   n=120;  * Number of payments applied to the amount while the interest
             accrues at the given rate *; 

   npay=mort(amount,  payment, rate/12,.);
   put npay= dollar10.2;   * Result indicates this payment would reduce the
                             balance to $0.00 after npay=360 payments *;

   balloon=mort(.,payment, rate/12, npay-120);
               * Compute the balloon amount by computing the
                 amount that would be equivalent to 240 payments of
                 725.07 subject to the 0.07875 interest rate *;
   put balloon= dollar10.2;

   cumulativeprincipal=amount-balloon;
   put cumulativeprincipal= dollar10.2;

   cumulativepaid=payment*120;
   put cumulativepaid=dollar10.2;

   cumulativeinterest=cumulativepaid-cumulativeprincipal;
   put cumulativeinterest=dollar10.2;
   run;

The previous statements produce the following output in the SAS log:

npay=$360.00
balloon=$87,497.07
cumulativeprincipal=$12,502.93
cumulativepaid=$87,008.40
cumulativeinterest=$74,505.47