According to BBC Sport, the deal includes an initial £23.9m fee plus £1.7m in potential add-ons, with the 20-year-old expected to undergo a medical.
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Methalie made 28 Ligue 1 appearances last season, scoring twice in his final three games as Toulouse finished ninth.
The left-back has emerged as one of France’s promising young defenders and has been a long-term target for Regis Le Bris.
Sunderland are keen to strengthen before their Premier League opener at Ipswich Town, while also preparing for their return to European competition.
The agreement represents a major investment as Le Bris continues to reshape his squad ahead of a demanding campaign.