I don't think Sambapos provides such an option...
You could make a product called deposit for 1,00 (when somebody pays $50,00 you type 50 then deposit), and map that deposit on a table. Then you can add products to the table. When the customer want to pay you cancel the product deposit. In the checkout you give as amount $50,00 then you print the ticket.
On the ticket you will have a total amount, and the amount already payed.