You're going to enter the sale of the home twice. So it's a hassle but not the worst one.
The first entry is the home sale just as it was supposed to be, You'll enter the interest that you received in the appropriate section and then you'll enter the home sale under sale of business property with the amount from the 1099-S as it is shown along with any closing costs, etc. Basically as if the transaction had gone smoothly. That covers what the IRS needs to see because of that 1099-S.
Then you will enter a second sale of the property. The sale date should be the date that you were made aware (a letter from a lawyer or something is good for a date) that your note on the property was worthless. If there is any hope for a partial recovery then the date of this sale should wait until you are sure (which it sounds like, in this case, you are). The sale price is zero and the basis in the property is the sale price of the earlier transaction.
When naming this sale it's a good idea to add 'default' to the sale name. This will result in a loss that will wipe out the earlier gain that you entered and show the reality of the situation at the same time.
@Tax Tiger
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