If your client has already paid for part of your overall invoice amount, then it is best to mention this somewhere on your invoice, when creating or reissuing a new invoice for the remaining amount. You can do this either within the description, or within the additional 'comments' section.

See examples below, for reference:

Below are examples of how both methods will look in the final invoice. On the left: if included within the item description. On the right: if included within the additional comments section.

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