GST ON DEEMED EXPORT

I think you are speaking about merchant exporter concept, where supplier supplies goods to Exporter who further exports the goods outside India. 

Refer Notification 40/2017-IGST(Rate).

Conditions of export and documentation are given therein. Where the transaction does not satisfy the conditions mentioned therein, the department could hold the supplier liable to payment of balance 11.9% tax. Therefore, suggest to always take an indemnification from the exporter on this aspect. 

Circular 37/2018-CGST provides details of going for refund under section 54(3)(ii) - Inverted duty structure where ITC has accumulated due to such reasons. 

For the exporter, where he procures goods at concessional rate he cannot go for refund with payment of tax, and only option is refund under without payment of tax.

Deemed exports is a separate concept u/s 147 read with notification 48/2017-Central Tax ,dt. 18-10-2017