I have another business that i often work with on various job. Some mine and some theirs. I have been using and older (6yr old) desktop version of MYOB on mac and had an invoice running to keep track of when i did work for them and when they did work for me. I would just go in and add a line item and amount when i did work for them and another line item and a negative amount when they did work for me.
This would automatically convert between an invoice and credit memo depending on the total amount. Is there a way to do this in MYOB essentials? Its generally something that we settle every 3-6m depending if the amount owing or owed becomes to great.