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.