There are four different scenarios due to which you may see negative values in your report-
1. Refund of a past transaction.
Refunds of past transactions get recorded in the current report. Say, a refund was initiated for a Virtual Card transaction on the 30th of April. It got credited to the account on the 2nd of May. So when you pull the report for May, this transaction will be counted in as a negative spend. If there are no transactions or transactions amounting to less than the refund in May, this will show up as a negative spend.
2. Refund credited to a different card.
Some merchants allow their customers to select a different card to process the refund. If the spend owner chooses a card different than the one which was used to make the original transaction, the refund will be recorded in the new card as a negative spend.
3. Refund was recorded before the transaction.
Sometimes the original transaction may get recorded after the refund transaction. When such delays happen, the report will show a negative spend until the transaction gets recorded.
4. Refund received is greater than the transaction amount.
In some cases, refund credited will be more than what was charged. Thus the report will show a negative spend.