Help! Missing Historical Documents. Senior Citizen

[Netminder3939]

I'm doing my 80-year-old father-in-law's taxes this year. I have to include a sale of his primary home in 2020. But no one has the purchase transaction documents from 20 years ago. The family generally knows the amount the father purchased the house for and the year, but not exact dates and numbers. The father has medical and cognitive issues, so he has no memory or records of the purchase. Zillow and similar online resources do not go back 20 years. The capital gains costs basis is well under the $250k threshold, but I simply do not know the exact numbers or dates to enter on the 1040.

What do I do? Do I get the father to sign an affidavit or something? 

[ColeenD3 · Accepted solution]

Estimate to the best of your ability. You may get an IRS letter, but all you can do is explain. An affidavit won't change anything.