The money was reportedly paid to the United States for the procurement of military equipment to fight Boko Haram.
According to a report by ThisDay, the money was paid to the United States for the procurement of 12 Super Tucano aircraft for the military.
The money was said to be a part payment - to avoid losing the opportunity to purchase the military equipment within the deadline.
Buhari, in a letter to the National Assembly, said he had to give the express approval for the withdrawal because the U.S government had given a payment deadline for the aircraft purchase.
The letter was said to have been sent to the National Assembly leadership on April 13 and it was received in the Office of the Speaker of the House of Representatives on April 17.
The President's letter reads, "I wish to draw the attention of the House of Representatives to the ongoing security emergencies in the country. These challenges were discussed with the state governors and subsequently, at the meeting of the National Economic Council on 14th December, 2017, where a resolution was passed, with the Council approving that up to US$1 billion may be released and utilised from the Excess Crude Account to address the situation.
"Subsequent upon this approval, we are preparing a comprehensive schedule of all the requirements for each of the security services for presentation to the National Assembly for consideration.
"It would be recalled that, for a number of years, Nigeria had been in discussions with the United States Government for the purchase of Super Tucano Aircraft under a direct Government-to-Government arrangement. Recently, approval was finally granted by the United States Government, but with a deadline within which part payment must be made otherwise, the contract would lapse.
"In the expectation that the National Assembly would have no objection to the purchase of this highly specialised aircraft, which is critical to national security, I granted anticipatory approval for the release of US$496,374,470.00. This was paid directly to the treasury of the United States Government.
"I am therefore writing, seeking approval of this House for the sum of US$496,374,470.00 (equivalent to N151,394,421,335.00) to be included in the 2018 Appropriation Bill, which the National Assembly is currently finalising. The balance of the requirements for critical operational equipment is still being collated from the different security services and will be presented in the form of a Supplementary Appropriation Bill, in due course.
"The Honourable Minister of Defence and other appropriate officers will be available to provide further details, as may be required.
"While thanking the Honourable Members for the usual cooperation, please be assured Mr. Speaker, the assurance of my highest regards."
The withdrawal is a breach on the Sections 80 (3) and (4) of the 1999 Constitution which states that:
"(3) No moneys shall be withdrawn from any public fund of the Federation, other than the Consolidated Revenue Fund of the Federation, unless the issue of those moneys has been authorized by an Act of the National Assembly.
"(4) No moneys shall be withdrawn from the Consolidated Revenue Fund or any other public fund of the Federation, except in the manner prescribed by the National Assembly."
At a meeting with the state governors last December, the National Economic Council had given the president the go-ahead to withdraw $1 billion from the ECA to fight the Boko Haram insurgency.
If due process were to be followed, the withdrawal would have required the approval of the National Assembly and the 36 state Houses of Assembly since the funds in the ECA belong to the three tiers of government.