https://help.tallysolutions.com/docs/te9rel61/Tax_India/gst/gst_discount.htm

GST Sales with Discount at the Item Level

While recording a sales transaction, you can easily enter the discount rate or amount for each stock item. To consider discounts for GST sales, you have to create a discount ledger and then select it in the invoice.

GST is calculated by considering the discount entered for each stock item after you enable the cash/trade discount option in the invoice. The corresponding tax values will appear in the GST analysis of items on the printed invoice.

Create a discount ledger

1.    Go to Gateway of Tally > Accounts Info. > Ledgers > Create.

2.    Set Type of Ledger? as Discount. Now the option Is GST Applicable? will be set to Not Applicable by default.

Record sales with discount

1.    Go to Gateway of Tally > Accounting Vouchers > F8: Sales.

2.    Select the Party A/c name and the Sales ledger.

3.    Select the stock item, and enter the Quantity and Rate.

4.    In Discount, enter the discount rate or the discount amount applicable for the stock item.

5.    Select the discount ledger. The total discount amount will be computed and displayed.

Note: You can alter the total discount amount, if required. For example, if you want to allow a maximum discount of Rs 50,000 in a bill, and your total discount amount exceeds this amount, you can alter the final amount. The discount will be apportioned according to the discount given at the item level.

6.    Select the applicable GST ledgers.

7.    Click A: Tax Analysis to view the tax details, and click F1: Detailed to view the tax break-up.

In the sales invoice, press Alt+P to print the invoice.