If a customer sends money (payment) without any invoice being sent by you, how do you enter it in Core? As a retainer or an invoice payment?
If you create an invoice for this pre-payment, you are essentially pre-billing for future revenue and will need to associate your employee costs with it later. A better option is to enter it as a retainer payment in the Payments screen. A retainer is more consistent with the flow of billing where you essentially don't consider the money as income until the work is completed. So in accounting terms, it is a liability till you bill it. Check the Core Help for details.