No, that's exactly what an audit is. They want to see proof. Sometimes you just need to send a letter to the IRS explaining your reasoning. If they don't agree, and you think you are right, then you should probably get a cpa. If they still don't accept and you want to fight their decision, then get a tax lawyer.
Having also been there, this was not my experience. The IRS sent me a bill for $1500 claiming I had taken an early deduction from my IRA. I had, but it was only to correct a mistake I made right before of contributing too much for a given year. I filed paperwork with my brokerage at the time to reflect this, but apparently the IRS didn't get that.
I replied to the bill with the relevant paperwork and a 1 page letter explaining what happened, and the IRS agreed I did not actually owe them anything. No lawyer required.