"open transaction" versus "installment agreement"

extraordinary cases in which the fair market value of the obligation cannot reasonably be ascertained."

says that every case is either an installment agreement or a closed transaction; treatment as an open transaction is NEVER allowed. He, the accountant, is required to determine a fair market value.

Actually I think I have it now. I found the actual regulation: "The fair market value of a contingent payment obligation may be ascertained from, and in no event shall be considered to be less than, the fair market value of the property sold (less the amount of any other consideration received in the sale). Only in those rare and extraordinary cases involving sales for a contingent payment obligation in which the fair market value of the obligation (determinable under the preceding sentences) cannot reasonably be ascertained will the taxpayer be entitled to assert that the transaction is 'open.'?

The "fair market value" referred to in not of the obligation, but of the property sold.

I guess you can always determine a fair market value (with varying degrees of certainty) for properties.