Sassuolo are holding out for £36 million (€40m/$47m) for Jeremie Boga, with Napoli having come closest and Rennes having also made a formal approach for the former Chelsea winger.
Napoli have made an offer of £27m (€30m/$35m) already with further add-ons being put forward, while Rennes have only been able to put an £18m (€20m/$23m) offer in.
Chelsea's approach for Rennes goalkeeper Edouard Mendy could see the Ligue 1 club earn up to £25m (€28m/$33m) for their current priority target with talks ongoing.
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Rennes manager Julien Stephan recently spoke out about his admiration for Boga as the Ligue 1 side look to secure a deal.