Kylian Mbappe, one of PSG's stars, came very close to signing for FC Barcelona. The Catalans reached a deal with Monaco to avoid him signing for PSG. An operation which took place one day before the window shut, but eventually broke down and saw Mbappe join PSG.
According to the documentary 'Kylian Mbappé - Hors normes (excepcional)', filmed by L'Equipe 21, Monaco and Barcelona reached a deal. There was nothing fixed between PSG and Monaco on the night of August 30.
Barcelona, angered by PSG's signing of Neymar, and with Ousmane Dembele's arrival sealed, reached a deal with Monaco involving a payment and Arda Turan going the other way.
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According to L'Equipe 21, everything was ready. Mbappe's plane was waiting in the airport, but it broke down and Mbappe ended up at PSG.