charitable contribution of decedent

No. It is a specific bequest from the estate.

Did he inherit the house and is writing his personal check and is not able to fulfill that bequest from the estate's resources? Or is the house considered part of the estate, which means the proceeds of the sale are the estate's, and he is in fact fulfilling that bequest from the estate?

In other words, what makes him thing he is donating his own funds?

If the house was sold in the Estate name and Federal ID number then he would have had to have an account such as a checking or savings account to receive the monies from the sale.  If he wrote a check out of that account for the $67,000, then the 1041 return would reflect the donation and all good.  If he distributed all the money to himself and there was no other money in the estate to make the contribution to the charity (which legally had 1st dibs on the proceeds) this could be interpreted as theft from the charity.

If he had transferred the house to his name and sold it, same problem as the charity had 1st dibs.

I am assuming the house sold in 2021.  If it were me, I would go and open up a checking account in the Estate's name and ID number and put back the $67,000+ a bit, write a check to the charity out of that account, wait for the check to clear, then close the account.  If there was no 1041 return filed for 2021, maybe open one with zeros, file a final one for 2022 with the contribution showing up in excess deductions upon termination maybe 😉