I sold my home in 2017 and had no capital gain. I financed it. The buyer paid it off in 2018. Tubotax says I owe tax on the full amount paying off the loan. Is this right

No. If you reported the sale of your home on your 2017 return and excluded the gain, you only owe tax on the mortgage interest you received from the buyer.  You will not report the portion of the loan payoff attributable to the principal loan balance.

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