You can accomplish this either by entering an Invoice, a Debit Note or an Adjustment...it really depends on whether you have entered the invoice already or not.

The thing to remember (if Customer is Open Item) is that the Document Numbers must match exactly (i.e. number for number, letter for letter - small case, upper case etc), otherwise the transactions will not get applied to each other and will stay in the Current Transactions until they are dealt with.

For instance, if you entered the Cash originally with a 'Document number' of CASH, then you must use CASH as the document number in the offsetting transaction, no matter if it's a debit note, invoice or an adjustment.

If you have not given the Cash and/or Invoice the same document number, then they will have to be cleared out individually, using the originally entered doc #.

It really depends on how you would rather see the entries show up on your reports or Customer Statements. Normally, if errors are made the Adjustment entry area is used, to identify the errors as adjustments rather than seeing them as Invoices, Debit Notes or Credit Notes on file, unless you have actual documents to support this.

Hope this helps,