The 23-year-old central midfielder has been identified as one of United's top targets as they look to strengthen their title-winning squad. But Manchester City are also scouting the Germany international after they were left impressed following trips to watch his teammate Robert Lewandowski.
Dortmund face a battle to hold on to their top stars this summer. Mario Gotze has already agreed a £31.5m move to Bayern Munich while speculation is gathering pace that Lewandowski is ready to call time on his career under Jurgen Klopp.
And Bender could be the next one out the exit door. A host of Premier League clubs have been linked with the young midfielder - including Arsenal.
Arsene Wenger has been looking for a replacement for Alex Song after the Cameroonian international left for Barcelona at the end of last season, and Bender is thought to be under consideration.
But the two Manchester clubs are expected to lead the chase this summer. Bender impressed during Dortmund's stunning Champions League semi-final first-leg victory over Real Madrid earlier this week, and scouts from both Manchester United and Manchester City have reportedly been full of praise for his performances this season.
United could be looking for a partner for Player of the Year contender Michael Carrick, and with Ryan Giggs and Paul Scholes approaching retirement, and Darren Fletcher and Anderson struggling for fitness, the need for a fresh face in midfield is growing.
City boss Roberto Mancini picked up Jack Rodwell and Javi Garcia towards the end of the last summer window, but neither has really hit it off at the Etihad Stadium so far.