The 21-year-old is set to return to France, where he began his career in the academy of FC Sochaux and became the club's youngest-ever player to feature in a professional match.
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According to Foot Mercato, both clubs have reached a full agreement for the transfer. Mayenda scored in Sunderland's opening Premier League match last season but struggled for regular opportunities, making 20 appearances overall.
The Spanish youth international will now look to revive his career under Franck Haise as Rennes aim to build on their recent resurgence and establish themselves as regular European contenders.