Fleck started his professional club football career at Rangers, and was on the books of the Gers from 2005 until 2012.
During his time at Ibrox, the Gers won the Scottish League twice twice and the Scottish Cup once.
The Scotland international joined Coventry City in 2012 and moved to the Blades in 2016.
The 28-year-old has developed into an important player for United over the years, and is now playing in the Premier League.
Fleck – who earns £8,000 per week as salary at the Blades, according to Spotrac.com – has said that he is open to returning to Rangers in the future.
Fleck told Sky Sports: “I must say first of all I’m very happy at Sheffield United. As long as that continues, great.
“If that [Rangers] chance came later on down the line, I would love it to be honest. I wouldn’t say unfinished business; I was a small part of a successful team back then and have great memories of the squad and that time.
“Looking back, I made the right decisions, going away and trying to build up my career again from scratch. There were some difficult times to begin with, moving away from home at a young age but I’m now at a great club in Sheffield United.”
Fleck has made 25 appearances in the Premier League for United so far this season, scoring five goals and providing two assists in the process, according to WhoScored.
The Scotland international midfielder made 45 appearances in the Championship for the Blades during the 2018-19 campaign, scoring two goals and providing 10 assists in the process, according to WhoScored.
The Premier League and the Scottish Premiership are suspended at the moment due to the coronavirus pandemic, and it is not clear when the summer transfer window will open.
United are aiming to finish in the Champions League places, while Rangers are looking to win the Scottish Premiership title.