Hollande and Buhari signed the letter of intention ahead of Saturday's summit.
It opens the way for a defence cooperation deal that will target terrorism and marine piracy.
It will mean French soldiers sharing expertise, such as reading satellite images, and will guarantee that, if they are prosecuted, they will face a court in France, not in Nigeria.
Hollande dubbed Boko Haram, which has declared allegiance to the Islamic State armed group, "the most murderous terrorist group in the world".
France, UK, EU announce aid
While hailing "impressive results" that have drastically reduced its field of operation, he said it was "indispensable that the international community do more".
Buhari estimated that 960 million euros are needed to "eradicate the causes" of the insurgency.
France has already set aside 25 million euros for cooperation with the four armies - from Nigeria, Cameroon, Chad and Niger - in the multinational force combating Boko Haram and 17 million euros for humanitarian aid.
Hollande promised that the French development aid agency would launch a project for the Lake Chad region, where the four countries meet and where Boko Haram has taken refuge from the offensives.
Despite British Prime Minister David Cameron's remark last week that Nigeria is "fantastically corrupt", Foreign Secretary Phil Hammond promised aid worth 57 million euros over the next four years, while the European Commission is to pay 50 million euros, up until now blocked at African Union level, to support the armies involved in the conflict and 1.7 billion euros in humanitarian aid over three years.
Hollande also promised that the Barkhane force, deployed in the Sahel region, could "intervene every time there is a terrorist risk or acts by the groups".
Multinational force not yet operational
The four-nation force has been theoretically established since last July but there have been growing pains.
Chad's President Idriss Déby called for better coordination and said it was urgent that it be operational before the rainy season.
"If nothing is done in three months all the zone controlled or claimed by Boko Haram today will be under water and we will have to wait till March 2017," he said.