Service tax on retention money

Hello, A bill for Works contract was raised inclusive of Service tax.Only 90% of bill value has been paid by the client,thereby keeping 10% as retention money.Now the doubt is on what amount do we have to deposit Service tax ie: 90% or the 100%.will the service tax on retention amount be paid when we receive the amount.plz do inform if there is any circular or notification or any case law for this.