I think the income received by the foriegn agency is not accrued/arised/deemed to accrue or arised in India/ as you mentioned that the agnecy has no branch in India. So the Income is not taxable in India. Please refer section 9.
For attracting taxing statute, there has to be some activity through permanent establishment and, if income arises without any activity of permanent establishment, even under DTAA, taxation liability in respect of overseas services would not arise in India.
case laws CIT v. Hyundai Heavy Industries Co. Ltd. [2007] 161 Taxman 191/291 ITR 482 (SC).
So i think in your case no tax is required to be deducted