The DolarCard -on both his physical and virtual version- is a secured credit card backed by the balance you hold on your account. This means that the amount that you have as balance in your account is the credit limit of your card.
If you have 100 digital dollars in your account, the credit limit of your card is 100 digital dollars.
Then, when you make a payment it would be discounted instantly from your balance, so if you pay 60 digital dollars then your credit limit would decrease to 40 digital dollars.