In the letter, the National Counter Terrorism Authority said the foreign spy agency had paid Rs 80 million to two activists of a banned outfit for the assassination of Saeed.
It asked the Punjab home department to ensure foolproof security for the Jamat-ud-Dawah (JuD) chief.
Saeed has been under house arrest in Lahore since January 30 under the Anti-Terrorism Act 1997.
The home department last month had extended his detention for another 30 days (till November 26) under public safety law.
The JuD has already been declared as a foreign terrorist organisation by the US in June 2014. The JuD chief carries a reward of USD 10 million announced by the US for his role in terror activities.