Lagos - Boko Haram was paid an equivalent of around $3.1m by French and Cameroonian negotiators before freeing seven French hostages this month, a confidential Nigerian government report obtained by Reuters said.
The memo does not say who paid the ransom for the family of seven, who were all released on 19 April, although it says Cameroon freed some Boko Haram detainees as part of the deal.
France and Cameroon reiterated denials that any ransom was paid. Nigerian authorities declined to comment.
Armed men on motorcycles snatched Tanguy Moulin-Fournier, his wife, brother and the couple's four young children, the youngest of whom was four years old, on 19 February while they were on holiday near the Waza National Park in north Cameroon, about 10k m from the Nigerian border.
They were believed to have been held in northeast Nigeria.
Nigerian Islamist sect Boko Haram claimed the capture of the family of Moulin-Fournier, who worked in Cameroon for French utility firm GDF Suez.
French President Francois Hollande at the time denied any money was paid when the family was released.
The Nigerian report suggests that 1.6bn CFA francs ($3.15m) was paid, but that right up until the last minute Boko Haram leader Abubakar Shekau had insisted on double that, before agreeing to reduce it if some Boko Haram members in Cameroonian jails were freed.