When I was working at X company, a vendor did not send me an invoice for material I was 100% certain we had received. I accrued the expense in the books and waited for the invoice to arrive. After a month had passed, I contacted the vendor for a copy of the invoice.

In researching the shipment, the vendor's accountant discovered that the paperwork was lost and was never turned in to the accounting department for billing. Even though my call to the vendor ended up costing my company some money, I still feel it was the right thing to do. It was a legitimate expense that we owed to the vendor and I would not have felt right about cheating the vendor.