Debit note in GSTR 1

In GST only Seller got right to raise CN/DN, where as you are purchaser no need to raise any DN/CN in GST portal.

1st you have accepted orginal invoice in gstr2a, inform to seller , for rejection details, based on your input he may issue CN in GST portal.

sir seller raise debit note how to file in gst return if i am file credit note site ask orginal invoice number how rectify this 

I file gstr 1 quarterly. Due to some mistake I have missed to add the details of the sales made in the months of July and August and I filled the gstr with the details of sales made in the month of September only. 

Can i now add the missed invoices of july and august in the next return? Can I add these invoices with the actual date invoice? Is there any penalty or interest for the same? Can I issue credit notes for all the invoices of july and august and issue new invoices to avoid any penalties?