The below info will guide you on finding prepayments that are attached to layaways. The below assumes you are taking in prepayments from the point of sale which will automatically assign the Layaway number to the reference field.
From the BusinessMind home page click on Sales
From the View Invoices section choose to create a new smart set
Name your smart set and select the columns you wish to view then create search criteria like the one pictured below
This will find prepayments assigned to layaways that have yet to be fully allocated to an invoice.