Excel Question:

Using ISNUMBER $($and an IF statement$),$ pull Extra Payment $($say D$5)$ or have the formula return $$N$/$A$.$

In addition to the ISNUMBER condition, here$$s the logic:

If PRIOR principal balance is GREATER than the PRINC paid $($not PMT$)$ plus the EXTRA PAYMENT, the cell should return the EXTRA PMT$.$

IF $($PRIOR principal balance LESS PRINC paid$)>0,$ then take this difference; otherwise $0.$ If none of this is true, $$N$/$A$$