I'm using 3 formula field.

One time commission, Monthly Commission and Grand total (total of 2 ) It is working but i need to ask what if

One time payment is Paid, How can i put logic to formula field that it will not add The onetime commission to the grand total and it will just show a 12usd instead of 232usd.. please see picture pelow

One time Commission is made up of 2 number fields 

Monthly Commission is made up of 2 number fields 

Grand Owe is made up of 4 Number Fields

This is what i want