While United will be cooking up a revised offer closer to the Bees' demands, a transfer is never complete until it's complete.
This is a club that knows such a reality better than most, a la the supposed capture of a talented teenage midfielder named John Obi Mikel in 2005 before Chelsea had other ideas.
Mbeumo could well be M16-bound, but until Brentford accept a bid, one of the Premier League's best forwards is on the market.
And Tottenham, among United's rival admirers, could be about to obtain the ultimate weapon to convince the Cameroon international to make a U-turn.
On Friday, Spurs announced the sacking of Ange Postecoglou just 16 days after he achieved Europa League glory against United in Bilbao.
Brentford manager Thomas Frank is reportedly the leading candidate to replace Postecoglou at the helm, with developments expected next week.
If Frank is considering jumping ship with Mbeumo, there could be a risk of the Dane attempting to convince his talisman to join him in a joint new venture.
United will be confident of sealing a deal, but Tottenham's bombshell announcement, which has reportedly sparked uproar among players, adds impetus to shake hands with Brentford.
Ignoring a club of Newcastle's trophy-winning scarcity for the allure of a global heavyweight like Manchester United is understandable.
But while Spurs may not hit those heights either, it's not outrageous to suggest a stay in London alongside a coach he shares an impeccable relationship with wouldn't appeal to Mbeumo.
The Brentford No. 19, who scored 20 goals and registered eight assists in the Premier League last season, is highly likely to stick to his guns and land at Old Trafford.
However, news of Postecoglou's dismissal and Frank's potential cross-London switch should act as a catalyst for United's negotiators to meet the Bees' asking price the second time around.
The last thing Ruben Amorim or the club needs is to be thrust into a transfer saga when they have such much more to address before the start of next season.