Switchover from regular to composite

Thanks Sir, so I will reduce sales return pertaining to sales done in regular period from the turnover in composite period. In my sale I had charged tax, should I reduce only sales amount excluding tax from composite turnover or I can deduct entire debit note value? Please reply. 

Also, how do I explain during audit because in GSTR-4 I am reporting only lumpsulm turnover?

Regards,

Santosh HR