Paying with Liven even if you do not have Brandollars left is still possible. And the good thing about it is that you receive rewards when you pay using your Liven credits and your fallback method.
*Note:*
If the Liven credit is on, the moment that your Brandollars is consumed to zero value, every time you dine in the restaurant where your Brandollars is maxed out, your Liven coins will be deducted too and when the Liven wallet is $0, then your fallback method will automatically be charged.
Scenario 1: When you have a Brandollars
Brandollars amount = $100 > Total bill is $100
Payment will be "Spend your $100 credit" until it is $0
Scenario 2: When you do not have a Brandollars but with Liven credits
Brandollars $0 + You have $50 Liven coins + Fallback method set up
The total bill is $58.95
Payment will be "$50 Liven credits" + $8.95 charge to the fallback method
Scenario 3: When you do not have Brandollars and no Liven credits
Brandollars $0 + $0 Liven credits + Fallback method set up
Total bill is $10.75
Payment will be a $10 charge to the fallback payment method
This should explain how payment works when your Brandollars run out.