Rocomamas cashier refusing to redeem voucher

We were at Rocomamas carnival mall My sister had a varsity vibe voucher, but the cashier told her to pay still even if the voucher is redeemed. Both the waiter and cashiers attitude was bad. They were not willing to prepare the meal at all. My sister had already redeemed the voucher and left it there. Most probably they used it for themselves. Because it does not make sense to redeem a voucher of 100, then pay the exact amount also.