Gst purchase entry

As per section 15 of gst, value of supply shall not include any discount which is given before or at the time of supply.
As stated by you that discount is shown in invoice ,so this is the case of discount at the time of supply.
So you have to record the purchase after discount.
For ex purchase 1000
dis 100
net 900
cgst @ 9% 81
sgst @ 9% 81
Total 1062

Then entry is
Purchase 900
input cgst 81
input sgst 81
To Vendor 1062

Due date for filing FORM GSTR -3B for the month of July 2019 has been extended to 22.08.2019. Notification will follow.

Income side if it's not reduce from taxable value in bill.