GST ON RETENTION MONEY DEDUCTED BY GOVT WORK

Hi Experts,
Contractor doing govt work (electrical dept) while giving contract amount to contractor govt will deduct some amount as Retention amount.
ex :-
Total Work value = 100(value)+ 18(tax) = 118
Govt payment = 60(value) + 18(tax) - 40(retention)= 78 (net payment)
**NOTE :- Gst TDS done for whole amount I.e 100.
In this scenario which amount we should take as turnover for the month?