I totally agree with you. If Brentford gets greedy and asks for more money, United should walk away. The offer is more than enough for a player of his quality. I also think Brentford is delaying a bit in accepting or rejecting the bid in order to see if any of the interested clubs will come up with an offer that's more than what United has on the table, but I don't think that's going to happen. They should just get it wrapped up.
If there are no other clubs who have the same interest as Manchester United then the second offer that Manchester United made should have been a decision to let him go because if it fails again then it is feared that Brentford will not get the same offer. Manchester United are not like they used to be because they consider their players and when they decide to switch to asking for other players then Brentford themselves will not get anything, because there are a number of players in the same position that Manchester United are also considering now.
As far as I know, Tottenham is also interested in Mbeumo, but it seems they are not making a higher offer than Man United, so in this case, Man United has a better chance to sign Bryan Mbeumo. Brentford have set their price, and Man United look set to meet it, so if Brentford are willing to ask for a higher price again and again, then it is likely that Man United will step down.
If Bryan Mbeumo does join Man United, then he will soon meet up with his compatriot (Andre Onana), and I think Andre Onana may have played a part in Mbeumo joining Man United.