The President of India has the power under Article 111 of the Constitution to return a Bill (except a Money Bill) to the Parliament for reconsideration.
Regarding the Post Office (Amendment) Bill, 1986, it was sent back to the Parliament for reconsideration by President Giani Zail Singh.
This action was taken due to concerns about the potential misuse of the bill's provisions, particularly concerning the interception of communications.
Therefore, Giani Zail Singh is the President who exercised this power for the specified bill.