You can configure Tally.ERP 9 to prompt you with a warning when the Cash ledger reaches a negative balance.
To activate Warning on Negative Cash Balance,
Go to Gateway of Tally > Accounting Vouchers
Set the option Warn on negative cash balance to Yes in F12: Configure screen.
Accept the screen to save.
Let us assume there is a cash balance of Rs.25000 and you need to make a payment of Rs.26000 towards Advertising expenses. This results in a negative cash balance.
The Payment entry is displayed as shown:
Tally.ERP 9 prompts you with a negative cash warning message and the credit cash balance in RED Colour. This will help you to decide whether to proceed with the entry or not.
More
Creating a Payment Entry with Narration for each Transaction