Twenty-two-year-old Walker joined FC United in October 2017 and made his debut away at North Ferriby United, in which he scored the opening goal in the 3-3 draw.
He has made thirteen appearances for the club, putting in consistently good performances, and subsequently attracting the attention from the Ammies.
Walker started his career at Bolton Wanderers, and made his first-team debut in January 2015.
He joined Bury on loan in August 2016, and was released from Bolton at the end of the 2016/17 season.
He went on to play for Stockport County before joining FC United.
Having previously been on a professional contract at Bolton, Walker has naturally been keen to return to professional football.
Fc United`s CEO Damian Chadwick told his club`s official website: “Although it is a huge blow to lose such a talented young player, we do not want to stand in the way of any player wanting to make a full-time living in football.
“Unfortunately, as he is not on contract at FC United we will not receive any payment from the transfer”.