To make an adjustment on a time or cost card, the accounting period must be open for Adjustments.
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Void and Reissue
Before we get into creating a true credit memo, you should first consider whether a void and reissue is the best approach. If the client requests a re-issue because of a mistake by your billing department, then you'll like want to correct that mistake rather than just correcting the monetary amount. For example, if time is entered against the wrong client, then issuing a credit fixes the money, but Projector still shows the time associated with the wrong project. When you look at operational data, like what project resource X worked on, it will be wrong. What you really want to do is void the original invoice, transfer the time cards to the correct client, and reissue.