I want to carry forward the amount from an unpaid invoice onto another (new) invoice.
Lets say that invoice 23 is unpaid and I am creating invoice 129 for the same customer. I know it is easy to add a new line with the unpaid amount from 23, however, this amount now shows on both 23 and 129.
What is the way around this? Do I just delete 23 or amend it?
That would really not be standard accounting practice. A sales invoice creates the account receivable. What you are talking about is a statement of account. This is available under Reports as a Customer Statement. You can set the period, and the report then shows the total outstanding balance due or balance owed for that customer.
While sending statements in lieu of invoices is something many businesses do, double-entry accounting doesn’t really involve statements. The obligation to pay is created once, and only once, by the sales invoice.
Thanks Tut,
I will have a look at Customer Statements and send in the debt collectors, lol.
It’s just a shame that some don’t realise their obligation to pay when they get an invoice.
Thanks again.
Another appropriate thing to do would be to forward a copy of the unpaid sales invoice with a letter requesting payment. Of course, if you impose a late fee or interest charge, that is a new transaction and provides an excellent opportunity to send the original invoice copy. 
Then, a Customer Statement would be especially useful, since it would show the mounting debt.