Gst payment received before issue of invoice & after supply

Ref.: 

Rule 5 of Invoice Rules and Sec. 31(3)(d) of CGST covers the situation of issuing Receipt Voucher in case of Advance Received.

Explanation 1 to Section 12 of CGST, Supply shall be deemed to have been made to the extent it is covered by the invoice or, as the case may be, the payment.

Doubt is :

1. What document shall be issued for Part Payment received after Supply of Services and Invoice is not issued yet (and 30/45 days have not been passed) ?

2. What document shall be issued for Last Payment received after Issue of Invoice ?

3. Whether such part-payment (i.e. received before issue of invoice but after supply of  services) be considered as advance since no invoice is issued till date ?

You may consider following hypothetical example to comment :

Date of Advance Payment Received : 01/07/2017 (of Rs. 5 Lakhs)

Date of Supply of Services : 10/07/2017 (of Rs. 10 Lakhs)

Date of Part Payment : 15/07/2017 (of Rs. 2 Lakhs)

Date of Invoice : 25/07/2017 (Invoice Value Rs. 10 lakhs)

Last Payment Received : 30/07/2017 (3 Lakhs)