Yes, it is possible to configure a bank account as a liability in Odoo when the balance is negative. You can achieve this by using the "Contra Account" feature in Odoo.

Here's how you can set it up:

1. Go to the Accounting app and select Configuration > Chart of Accounts.

2. Create a new account with type "Liability" to represent the overdraft on the bank account.

3. Go to the bank account that you want to configure as a liability and select the "Account" tab.

4. In the "Overdraft" field, select the liability account that you just created.

5. Save the changes to the bank account.

This way, whenever the balance on the bank account becomes negative, it will automatically be recorded as a liability in the balance sheet. Additionally, the offsetting mechanism is disabled, meaning that the negative balance cannot be offset by positive balances in other bank accounts.

Please note that this is only supported in Odoo version 15 or higher. If you are using an earlier version, you may need to upgrade to the latest version to take advantage of this feature.