The Catalan side had made United an offer for the French international, who is understood to be keen on a move to the Camp Nou this summer.
However, with little time remaining to find a replacement, United have shut the door on any potential move, causing Barce to re-centre their focus on de Jong.
According to Spanish publication 'Sport' on Wednesday, the club will lodge an offer of €50m with the Dutch side in the coming days, with that figure being what Ajax demanded when Barca first registered their interest back in July.
Despite Ajax being reluctant to lose any of their young starlets this summer, they know than an offer of that size would be very difficult to turn down.
Ajax are currently battling to ensure they return to the Champions League proper this season as they look to progress through the qualifying phase, something which could be key for Barca, with the Amsterdam-based side thought to be more open to negotiating after qualifying is completed.
Whatsmore, it had been suggested that de Jong could remain in the Dutch capital on loan until next season, but the aforementioned reports claims that Ernesto Valverde wants the player in and around the first-team squad this season in order to give him time to learn from the likes of Sergio Busquets and Ivan Rakitic.