When you say "face value", do you really mean the remaining balance on the note at the time of purchase? Or, is the original loan amount (the face value) $90K while the remaining balance at the time of purchase is less than $90K?
If the borrower pays off the note when the balloon comes due, the first $72K you receive is a return of capital and not taxable. The difference between the $72K you paid for the note and the remaining principal balance on the loan is not a return of principal, it is additional interest and will be taxed as such at your ordinary income tax rate.
If you sell the note for more than you paid for it but for less than the principal balance remaining on the note, then the difference between your purchase price and your sale price is a capital gain and taxed at the capital gains rate that applies to your tax bracket rate.
This is a general overview. The actual calculations are a little more complicated. Consult your CPA for more specific guidance.