Diafra Sakho is wants to leave West Ham because he feels undervalued compared to the club's other attacking players, according to Sky sources.
Sky Sports News understands Sakho, 28, is currently earning £30,000 a week at the London Stadium, while other forwards in David Moyes' squad are on between £80,000 and £110,000.
The Senegal striker believes he was promised a new, improved contract which is yet to materialise.
Sky Sports News reported last week that Crystal Palace's proposed move for Sakho was in doubt, despite the Eagles having agreed a fee of between £10m and £12m.
Swansea, West Brom, Brighton, Bordeaux, Rennes, Nice and Marseille are among the sides interested in a deal for the striker, according to Sky sources.
West Ham's assistant manager Alan Irvine said Sakho missed a training session on Monday because he was injured.