The company says one line on their house account statement is not theirs. What can I do?

Open Settings → House accounts and click the account. The statement lists every sale with its order number, every payment and every adjustment, newest first. Take the order number to Find a sale on the POS: it shows what was on the sale, when, who took it and how it was paid. If the sale was rung to the wrong account, use Adjustment (signed) on the account page with a minus amount and a note explaining why, and put the same sale on the right account the same way. Every adjustment carries your note and stays on the statement. Download CSV gives the company the corrected version.