The Serie A giants have placed a sizeable price tag on Higuain to dissuade any interested parties.
Following his £34.5m move from Real Madrid in 2013, the Argentine has grabbed 35 goals in 69 games which has sparked Premier League interest.
United manager Louis Van Gaal will oversee a major rebuilding job in the summer after securing Champions League football next season and signing Higuain would prove a useful addition.
Arsenal have also registered an interest in the striker with manager Arsene Wenger looking for some attacking reinforcements to help mount a serious title challenge next season.
The Naples club finished 5th in the league after a 4-2 home defeat against Lazio, where Higuain missed a penalty.
Higuain levelled the scores after Lazio took a 2-0 lead but goals from Ogenyi Onazi and Miroslav Klose in the 90th minute sealed the win for the away side.
Manager Rafael Benitez, who is widely tipped to become the new Real Madrid manager, needed a victory to ensure Napoli secured Champions League football next season.
Written by Jack Wynne – @JackRobertWynne