Get your taxes done using TurboTax

@RobertG's reply is pretty good. I can try giving another example. But first, a disclaimer that I am not a tax professional of any kind.

Do you know any other information about the installment sale? For example, do you know the sale price? If not, that's OK, I didn't know mine either, but I will tell you how I came up with a value.

Second question, do you know the total amount of payments received in prior years? I believe you should have this information as it is what we've always had to put down on Line 23 of Form 6252 in years past.

Next questions, is this the first year you are receiving payments? Or is this the last year that you'll be receiving payments? Or are you somewhere in between and you are now having to go through this process after the IRS changed it for tax year 2019?

If this is neither your first year, nor last year, then I can probably help, as that is the situation I'm in.

So, what I did first was to make a best guess at the sale price. I was able to do this knowing/guessing a few other pieces of information. First, I know the monthly payment amount I get. In my case $1,035.88. Second, the interest rate of the loan. On one of the forms I get each year from the buyer it says, "The minimum interest rate of 7.5% still applies." And lastly, I believe the payments are for 30 years. So all I did was go to an online mortgage calculator and set the term to 30 years, the interest rate to 7.5% and then started changing the loan amount until it showed a monthly payment of $1,035.88. In my case, that came to $147,944, or that was close enough.

Enter the sale price value on Line 5, Line 7, and Line 18.

Next, given that you know the Gross Profit Percentage, we know that the Gross Profit Percentage comes from dividing the Gross Profit (Line 16) by the Contract Price (Line 18). In my case, my Gross Profit Percentage is 0.9442 and my Line 18 is the sales price of $147,944. Therefore, Line 16 would be equal to $147,944 * 0.9442 = $139,688. This value is also used on Line 14. And since the instructions for calculating Line 14 say, "Subtract line 13 from line 5", we can compute Line 13 as our sales price minus the value we calculated for Line 18. In my example, Line 13 is equal to $147,944 - $139,688 = $8,256. You can then use that value as your Cost Basis on Line 8 (as well as on Line 10).

Here is a screen shot of that section of my Form 6252.

You may notice that a couple of the numbers are slightly off from the calculations I gave above. That is just some rounding error due to me not rounding the percentage. The point is, you're numbers don't have to be perfect either, it should be OK, the IRS isn't going to penalize you for it.

However, do be sure to review the forms before submitting your return to the IRS. Compare them to last year and make sure there aren't any big differences. The values you enter for the sales price and cost basis, no matter what they are, shouldn't change what you would otherwise typically pay in taxes. The IRS is just trying to gather more information. But again, that statement and this example are only if this is not the first year nor last year of the installment sale.

As for how to get all of this into TurboTax, I do not know. I struggle/fight with it each year. Review RobertG's post and screen shots again and see if you can get it.