Legality of making a sale before purchase under gst/indian law

Yes, you can go ahead.

As you are going to pay GST when the liability arises (Invoice is issued) and would claim ITC on purchases when you receive the goods, it is in compliance with the provisions of GST Act.

However, your customer can't claim the ITC in Sept month as he has not received the goods even though he is in possession of the invoice and tax has been paid.

You can make the sale but you cannot transfer material since you don't have it. Therefore inventory cannot be negative.