TDS on Retention Amount

Hiii

In a construction contract, I am a contractor and i got a bill from sub-contract Bill amount is Rs. 5,00,000/- & i deduct Retention amount say 10% Rs.50,000/-(it can be settle after work complition), & I have to make payment of Rs.4,50,000/-, Than on which amount i should deduct TDS it is on 5 Lacs or 4.5 Lacs, please guide me, please also provide any circular, notification or case law in context of this. Thank you,

-CA. Ravi N.