How does the gift card works?

Guys,
I would like to enable someone to buy a gift card. For instance, if they buy a U$70 gift card and then use it to buy a U$50 product, they should still have U$20 credit.
How does that work? Will this extra U$20 be lost or will it be available for the customer?

Thanks,
Oscar

The person who knows the gift cart coupon code will be able to use this extra $20 later