You can give discount in tally as many ways as follows.
1) For each item discount (for this you have to enable separate column for discount option)
2) A lump sum amount for overall invoice total (for this you have to create a ledger “Discount” under indirect expense.)
3) A percentage in for overall total invoice.

Give discount in tally separate column option

From the Gateway of tally F11 and select inventory features ,
Select Separate column on invoice and set enable to “Yes”
give discount in tally-1
Now y a separate column will appear in sales invoice and purchase invoice for discount.
For Example, You are selling a mouse for SR. 25/- per pieces and discount allowed 10% then, to your clients enter the qty rate as 25 and enter discount rate as 10%.
Note : in the above procedure You can able see the discount rate but not the discount amount
it is beneficial if you are taking a item wise profit reports.

Give discount in tally lump sum amount as a separate entry

This case you need to create two ledgers for “Discount Given” under group direct income and discount given under group direct expense (sales discount)
While entering the sale and purchase voucher , first enter the item names , quantity , rate and amount
after finishing stock item details simply press the enter key once to leave a blank line and select the ledger name Discount given ledger (in sales) and enter the discount amount with (-) value.