If you sold investment property at a loss, then you may have had a long-term capital loss, which would be reported on schedule D. Depending on the amount of the loss and any other schedule the capital gains or capital losses, you may have deducted that loss in the year you sold the property or you may have a carryover deduction for future years. If you did not report a capital loss for the year that you sold the property you should go back and review your tax returns for that year.
Separately, the interest on the seller financed loan is taxable, no matter how much the loan is for and no matter what the selling price of the property was. That interest is simply taxable income.
