Enter the booking
Click on the 3 points at the "Price" sector
Select "Use balance in favor of the client"
You will be shown all the balances in favor of the client according to each booking, in case they do have.
Select the balance in favor of the booking you need to use and click on add
You will be able to see the details at the "Price" sector
In case you do not have permission to proceed with this transaction, inform your administrator so he can enable you the "Can use customer balance" permission.