When we consider assets for a balance sheet or other financial statements, the Account Type is not important, but if the line item is an asset or liability.
This means if you had an account open where you actively owed money to another entity, it would be considered a liability.
Similarly, if you had a Credit Card balance that was negative, where the financial institution owed you some value back, it would be considered an asset.
EDIT: Citations would probably be useful for something like this.
A balance sheet is an accounting tool that lists assets and liabilities. An asset is something of value that is owned and can be used to produce something. For example, the cash you own can be used to pay your tuition. A home provides shelter and can be rented out to generate income. A liability is a debt or something you owe.
Liabilities are debts you owe
[The company] has the oblication or liability to repay the bank for the courtesy extended to [the company].
Negative cash balances are reported as a liability
At a more specific level, the negative balance term commonly refers to the checking account, where you have a negative balance if you have issued checks for a larger amount of cash than is available in the checking account. In this situation, create a journal entry to shift the amount of the overdrawn checks into the accounts payable or a similar current liability account; doing so reduces the balance in the checking account to zero, and properly displays the overdrawn amount as a current liability.
Negative balances are recorded as liabilities