You sold $550.00 worth of goods to a customer but sent him and Invoice of only $500 by mistake under Invoice number IN2431.
Note that you have not sent out the Invoice yet, you can easily amend the Invoice in Ezy Invoice. If you have sent it out already, you could follow up with a Debit Note for $50/-.
In the body of this Debit Note, you could for example key in the text “Being short billing in our Invoice Number IN2431”. When customer pays you for $550.00, knock it off in your AR (Accounts Receivable). Knock-off against both the Invoice IN2431 and the Debit Note just created.