GST on outward services in RCM

I'm home loan service provider for SBI , on outward services GST is applicable @ 18 % in Reverse Charge mechanism.
but if I raise bills of 100 Rs SBI use to retain 10% of total bill amount that is 10 Rs for 24 months.
on which amount should I apply GST for particular month ?
I. e. on 100 Rs or on 90 Rs .
& Also there is 10% deduction of TDS @ 10 %