Expense report number postfixed with "-1"

When an user submitted an expense report, his expense report number was discarded and a new expense report was generated with "-1" postfixed. He came up with the following questions.

Question: Why does my original expense report have a "-1" after it?
Answer: The expense report was short-paid by Accounts Payable – probably for lack of a receipt. Please contact your AP department and submit receipts where required.

Question: Why do I have a new expense report number?
Answer: The original expense report included two or more items. At least one item was short-paid by Accounts Payable. The original expense report was partially paid. The short-paid amount was given a document ID with a “-1” suffix. A new expense report was created to capture the amount of the short payment. Ultimately, there will be two payments which are associated with two different expense report numbers.