Value of Service in GST

I am Supriya (94 Points)

07 February 2018  
If a service is received from USA (import) for $100 (at Ex rate:64 i.e. Rs.6,400) on 20-7-2017.
A part payment is for $90 (at Ex rate:63 i.e. Rs.5,670) is made on 18-8-2017 and exchange gain earned is Rs.90.
GST has to be paid on RCM basis.What will be the Value of Supply?
Is it Rs. 5,670 or
Is it Rs. 5,760 (i.e. 5,670+90)