Niang spent last season on loan at Watford, who opted not to buy the Frenchman, and he is now surplus to requirements at Milan after Porto striker Andre Silva and Franck Kessie (Atalanta) were recently acquired by the Serie A giants.
Milan, taken over last season by Chinese investors, are also hoping to sell Colombian striker Carlos Bacca to Villarreal.
Niang has completed three loan spells during his time with the seven-time European champions.
He joined Milan in 2013, but was sent on loan to Montpellier then Genoa in the second half of the 2013-2014 and 2014-2015 seasons respectively.
According to Gazzetta dello Sport, Niang will undergo a medical with the Russian club in Rome on Friday before signing an 18m euros deal that could see him earn 3m euros a season.
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