Please follow the steps mentioned below to give discount on before GST amount:
Step 1: Go to Bills >> Sales >> Click on 3 dots for the invoice for which you are looking to apply discount
Step 2: Click on Edit
Step 3: Click on Edit for the products to which you are looking to apply discounts
Step 4: Click on the Discount type option
Step 5: Select the discount type as Unit price/Net amount to apply discount before tax amount
