In MYOB BusinessBasics, you can track work on retainer by creating a customer credit for the retainer and then applying the outstanding payment to invoices you record for the customers who retain you.
When you're ready to bill the customer for sales activity, you'll create an invoice and then apply the customer credit to the invoice using the Settle Returns & Credits window.
Please note that when you record a blank invoice to create a customer credit, your default checking account for customer receipts is debited and your accounts receivable account (usually an asset account) is credited.
To track work on retainer using a customer credit (includes Recording the retainer amount and Applying the customer credit to the customer's balance)