sales order with free voucher given to customer

Dear SAP experts,

my client wants when sales order is created with certain amount e.g. 100$
a multiple external gift vouchers will be given to the customer with 10$ amount
how can i maintain this is SD with the external voucher so I can track the voucher number with the sales order?
i will be higly grateful if you could guide me with technical configuration.

Best Regards.