I don't think it works that way ( and please correct me if I'm wrong). United doesn't have to agree anything with barca... not even the player has to agree anything with barca. When a team pays a player's clause( at least in spain ) there has to be no agreement between clubs. in this case, united would just have to show up and say 'Barca, we are paying thiago's caluse" and barca wouldn't be able to do a thing but accept the money. That's the reason why caluses exist. The only way united wouldn't be able to get Thiago is if thiago refused to accept united's offer. If Thiago and united have accepted to come together, there should be nothing stopping united from getting him.,
Nah its not quite that simple unfortunately otherwise it would already have been done, the clause exists but for non national clubs trying to use it (clubs outside of spain) its not as simple as pay it and get the player, see the Bayern Munich deal for Javi Martinez last summer, there are things like taxes that have to be added on and if the 'selling' club is a totally unwilling seller it all gets very hostile and complicated, it took Bayern weeks to sort it all out, hence why weve been trying to negotiate with Barcelona to keep good relations and make things much more simple, its not as simple as just paying the clause for a foreign club anymore, somtimes they even force the departing player to pay some amount of the taxes, i think thats what Martinez had to do to force his move last year.
If Real Madrid want Thiago then it isnt anywhere near as complicated, they are in that position whereby they pay the clause and if Thiago agrees to go Barcelona can do nothing about it, but thats why i dont see Madrid as a likely option, i cant believe Thiago would do that, wanting to leave for more game time is one thing, but hes not guaranteed that at Madrid anyway and it would taking a giant shit on the people whove made him what he is today, and the fans, it would be 10 times worse than what Figo did.