I had this happen to me a few years ago. I got a letter that I owed more than 200K I called my accountant very worried. I always give the government their due.
He explained that these initial letters come out of machines and the machine did not pick up a real estate transaction. So he wrote letters and told me not to panic. Took almost a year of letter writing before things were cleared up and I got the letter that said I owed zero. That was a relief.
So get to a good accountant if you don’t already have one.
Another hint:
IF you are audited for ‘contributions’-—only take your records for your contributions. NEVER take anything else to an audit. IF they see you with an entire box, they can get really inquisitive right there & then.