Your Mezu account Balance is always visible at the top of the screen where you can transact so you'll always be aware of how much money you have available.
The balance is made of funds you've loaded or received. It can also have promo funds that you redeemed on a campaign and funds that expired from transactions you attempted.
When you want to Give/Pay to someone and there's not enough balance, that amount will be taken entirely from your default payment method. For example, you want to Give or Pay someone $10 but your balance is only $7. The $10 will be taken from your default bank account or debit card and your available balance will remain at $7.
You may also tap your Balance to be immediately redirected to your transactions.