The Argentine centre-back was heavily linked with a switch to Old Trafford over the summer, but David Moyes eventually opted against pushing through a move during a disappointing transfer window.
However the Scot has remained keen on the South American enforcer and is now prepared to push through swoop, but not until the end of the season.
Benfica have always maintained that he is not for sale, yet it is understood that the player himself is keen to make the move.
United were initially looking to secure the 27-year-old’s arrival this month, but by holding off for another six months they will secure a discounted fee.
Garay will set the Red Devils back around £8m, which is seen as a reasonable sum for the man being hailed as Rio Ferdinand’s long-term replacement.
The Benfica star spent a disappointing spell at Real Madrid, but has since thrived in Lisbon and developed a reputation as one of European football’s top talents.