The 19 year old has been on trial at Andy Ritchie's club for the last few weeks and netted in the Tykes reserve victory over Sheffield Wednesday last Tuesday night.
The Australian born Left Winger is a product of the Blades infamous Academy system and is considered as 'one for the future' after being signed on a free transfer from Aston Villa last summer after being released by the Midlands club.
Nix has played just two first team games for the Blades, both in the Carling Cup against Shrewsbury (round 2) where United just edged through on penalities after a 0-0 draw and, he played in the following tie at Reading in a 2-0 defeat. Nix could make his Reds debut next Saturday when they face Swansea City at Oakwell.