My client got the mortgage interest audit.  Turns out they over-reported their interest since the loan is more than $750,000.  We expected them to adjust the taxes and send a bill.

Instead they sent a response saying they are not allowed to deduct any interest at all for 2021 because "they did not establish the amount shown was interest expense and was actually paid".

What documents can we provide to prove mortgage interest was legit and it was paid (besides the 1098 form and maybe the closing statement from the purchase)?