Entering a loan interest payment that doesn't reduce the loan balance (Q Mac)

MJT852

When entering a split transaction for a loan payment, I have one category for the "loan principal" amount paid and a second category for the "loan interest". For example, a total payment of $100 is split into two categories:

Loan Principal paid: $60

Loan Interest paid: $40

When the category splits are entered in this way, my loan balance showing on the register is reduced by $100, but the loan balance should only be reduced by the principal amount paid ($60). The interest paid shouldn't reduce the loan balance. How do I correct this problem?