The easiest way is to use the prepayment function.

1. Go to Tasks > Payments to issue the 50% down payment. Before you enter any detail lines, check the Prepayment box. Then you can enter a description and amount. The GL account defaults to AP but you can change it to your prepaid asset account. (I'll explain more about that at the end.) This payment will show up like a credit memo on the vendor's account.
2. When you receive the inventory, enter the invoice like normal in Tasks > Purchase/Receive Inventory.
3. When you are ready to pay the invoice, go to Tasks > Payments, enter the vendor ID, and select both the prepayment credit and the invoice for payment. That will leave you with remaining balance of the invoice as your check amount.

Regarding the GL account in step one. Sage 50 automatically sets the GL account to accounts payable because the prepayment will show up on the vendors account like a credit memo. So if you set the GL account to anything other than AP, your Aged Receivables reports will not match the balance in the AP account. The larger the prepayments and the longer they will be on your books, the more justification there is for using a Prepaid asset account. So if you need to use one, go ahead and change the GL account for the prepayment to your Prepaid account, then when you print an aged payables report go into the options and filter it by AP Account ID. Set the filter to your AP account to get a report that ties to your AP account, then change the filter to your prepaid asset account to get an aging of all your prepayments. The total of the two reports should match the total on an unfiltered aged payables.