The 27-year-old joined City for £27million in 2011, but has always struggled to hold down a regular starting spot.
Of his 70 Premier League games for City, he has started just 36 of those - but still has managed 28 Premier League goals.
City boss Roberto Mancini is a fan of the Bosnian, but has conceded that because of his lack of starts he could very well be sold this summer and Dortmund are favourites for his signature as they are due to sell Robert Lewandowski.
General manager Michael Zorc has revealed that the club are interested, but denied a deal was already done - admitting there could be an issue with the "economic aspect" of any deal.
"There should be little doubt about Edin Dzeko being a top-class striker," Zorc told Sport-Bild weekly.
"He is a great forward and a player one should be keen on. However there is also still somewhat of an economic aspect.
"It would be down to pure maths because Edin Dzeko is under contract at Manchester City beyond the end of the season."