advances receipt from customer

If the advance received and subsequent sale is done in the same month means no need of declaring the advance, actual sales only need to declare in GSTR1 and hence  Rs. 30,000 is taxable.

In my opinion GST will be charged on 40,000/-

No duty charged on Rs.30000/- only as sale is equal to that amt only. 

You received advance of Rs.40000/- not sold goods in respect of this. 

Because you sale only of Rs.30000/- & raised bill in respect of that sale only. So,GST charged on that amt only.

dear vikas in this case on recipt of payment it is mandatory to generate receipt voucher serially which would be of rs 40000/-

& then at the sale time generate tax invoice of Rs 30000/-, then generate Refund voucher for RS 10000/- & payment  of gst will be automatically upon rs 30000/-. this is law prescribed procedure if followed actually

But Refund voucher would be generated if no further sale with the customer otherwise excess 10,000/- will be adjusted in next sale.