I use the Write Cheques function>Payable to: the subcontractor. Enter 'Mid-month Advance in the Memo field. In the bottom section of the cheque, select the Expense tab (not Item) and select Accounts Payable as the account. There are no taxes on an Advance (or Prepayment), enter the amount of the Advance. In the Customer/Job column, select the Vendor (Subcontractor). Complete any other details you may need. This will create a Credit in the Sub-Contractor Account.
When you get the bill from the subcontractor for the month, enter the bill with applicable taxes, apply the Credit and pay the balance of the bill.