sold installment contract to a bank at a loss

from IRS pUB 537

A disposition generally includes a sale, exchange, cancellation, bequest, distribution, or
transmission of an installment obligation. An installment obligation is the buyer's note, deed of
trust, or other evidence that the buyer will make
future payments to you.
If you’re using the installment method and
you dispose of the installment obligation, generally you’ll have a gain or loss to report. It’s considered gain or loss on the sale of the property
for which you received the installment obligation. If the original installment sale produced ordinary income, the disposition of the obligation
will result in ordinary income or loss. If the original sale resulted in a capital gain, the disposition of the obligation will result in a capital gain
or loss. If the original installment sale resulted in
a section 1231 capital gain (or loss), the disposition of the obligation will result in either a
long-term capital gain or an ordinary loss.

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in my opinion:

 in your situation, you originally sold it for $X (downpayment + principal on installment obligation) and had a gain of $Y of which a portion was excludable.  so now you have to recompute the gain by using the principal received from the buyer and the bank as $X. your taxable gain is now reduced by the difference in proceeds but not below zero. so that was the gain to have been reported on the installment basis. over the years you reported a taxable gain of $A.  if your revised taxable gain is say $20 and to date you reported $15 you have a taxable gain of $5. if the revised taxable gain is $0 and you have reported $15 you have a capital loss of $15. 

if there was no taxable gain originally the loss on the sale of the contract is not deductible.  you use $150K as the sales price and for basis you use whatever amount is needed to report the revised gain or deductible loss.

if the property was ever a rental - see a tax pro