According to journalist Matteo Moretto, Rayo Vallecano will acquire 80 per cent of Mendy's rights, with Real Betis retaining a 20 per cent sell-on stake in any future transfer.
The 21-year-old's loan agreement, signed in August 2025, contained a clause making the purchase mandatory if Mendy reached 25 official appearances with a minimum of 45 minutes played in each. He finished on 24.
Rayo's sporting director David Cobeño had made clear weeks earlier that the club intended to exercise their separate, optional purchase clause regardless of whether the automatic trigger was met.
"He had a mandatory clause based on appearances and an optional one for the club. We are delighted with his performance. This is his first year as a full professional and he has shown a very high level, with occasional lapses in some matches, which is normal for young players. Our idea is to buy him and make him a fundamental piece for the future."
Mendy, born in Guédiawaye, Senegal, joined Betis's reserve side from Paris FC and made his breakthrough at Vallecas after a loan move that initially looked like a developmental step. Instead, he became a first-choice centre-back, partnering alongside the Rayo defence under head coach Íñigo Pérez and earning his first senior call-up to Senegal's squad — featuring as they won the Africa Cup of Nations in Morocco.
His rise has not gone unnoticed elsewhere. Mendy negotiated with PSV Eindhoven and Rangers earlier in the process before those moves fell through, and Nottingham Forest, Benfica and Atalanta have all been mentioned as admirers, according to MundoBetis.
For Betis, the sale represents their first of the summer window, adding to variable payments already received from Jesús Rodríguez's move to Como 1907. The Andalusian club face their own financial planning challenge ahead of Champions League qualification, and further departures — Sergi Altimira and Nelson Deossa among the names mentioned — are expected as the club works toward balancing its accounts for the 2025-26 season.
At 1.87m tall and left-footed, Mendy represents exactly the kind of asset Spanish clubs increasingly look to develop and sell at a profit. For now, Rayo have him. The 20 per cent Betis retain means this story may not be over.