First, no journal entry will be involved, either for invoicing or receiving payment. See this Guide:
Second, unless you have a contract or sales agreement that commits the customer to paying the entire 12,000 regardless of whether any work is performed, issuing a sales invoice in advance is not sound accounting. A sales invoice is a demand for payment for goods/services delivered. If you are using accrual basis accounting, you will immediately recognize 12,000 of income, on which you will have to pay taxes, without having earned anything. See this Guide:
Third, Manager has no current capability for project accounting, so an “exact match between returns and costs” will not be possible on a project basis, if that is what you meant. But you can create Profit and Loss Statements for any defined period of time. See this Guide:
The appropriate way to do what you want is to issue a sales invoice when the first month’s worth of service has been performed. Or, if the sales agreement stipulates payment in advance to cover a month of work, you could issue it at the beginning of the month. But the point is that, before issuing a sales invoice, the obligation for the customer to pay must have been established. Otherwise, what you are asking for is a deposit, which should be done with a Sales Quote (possibly retitled as a pro forma invoice). See these Guides:
Once you have created the first month’s sales invoice, set up a recurring sales invoice. See this Guide:
That Guide explains how to set up a recurring invoice from scratch. You can also use the Copy to... function from your first sales invoice for this customer. See this Guide: