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How to reconcile final payment on installment sale for IRS form 6252?
Reading IRS Regulations section 15a.453-1(c) more closely, I see that this was a contingent payment sale with a stated maximum selling price. Therefore, at the time of filing prior years, there was no way of knowing the final contingent payment would not be reached and thus the prior years were filed correctly and should not be amended. Those regulations go on to give an example of a sale with a stated maximum selling price where the maximum was not reached due to contingency and states that in such cases: When the maximum [payment] amount is subsequently reduced, the gross profit ratio will be recomputed with respect to payments received in or after the taxable year in which an event requiring reduction occurs. However, in this case, that would result in a negative gross profit ratio on line 19 of form 6252 which Turbo Tax reports should be a non-negative number.  Looking further in the regulations, I found an example which relates to bankruptcy and a resulting loss in a subsequent year: For 1992 A will report a loss of $5 million attributable to the sale, taken at the time determined to be appropriate under the rules generally applicable to worthless debts. Therefore, I used a gross profit ratio of zero on line 19 and entered a separate stock sale not reported on a 1099-B as a worthless stock on Form 8949 as a capital loss based upon the remaining basis in the stock sold in an installment sale.  I also included an explanatory statement with my return to the IRS stating: In 2008, I entered into an installment sale of stock. The sale was a contingent payment sale with a stated maximum selling price. The sales price did not reach the agreed upon maximum sales price due to some contingencies not being met. According to the IRS   Regulations section 15a.453-1(c) my basis in the stock remains at $500 in 2012 after the final payment. Rather than using a negative gross profit ratio on line 19 of form 6252, I'm using a zero ratio and treating the remaining basis as a schedule-D loss similar to worthless stock since the sale is now complete and my remaining basis is no longer recoverable.