We mere posters can't know anything for sure, but we should be confident United management know what they're doing. The bid has to be real and has to have been thought through. So you have to be right that United management know something we don't either about Fabregas's desire to leave and/or Barcelona's willingness to sell.
With respect to the right of first refusal, there is reason to believe Fabregas might want to move back to Arsenal. We here on the caf can't know what he wants, but what we do know is that it could be very difficult for Fabregas to return to a club he said adios to once before. There's also the question of whether Arsenal even need Fabregas, in light of the solid form of Cazorla (NB: I think he's overrated but gooners love him), the promise of Wilshere, solid depth in midfield behind those two and Arsenal's needs elsewhere. So if our bid goes up to 40m -- and I would say Fabregas would be worth that much -- one has to wonder whether Arsenal can stomach matching that price tag. I'd say no.
There's also the question of whether Fabregas would want to rejoin a club that has gone trophyless for 8 seasons and shows little hope of ending that drought this season. I'd say no.
Still, it's hard to see this one happening. Fabregas is a Barcelona man through and through and he's going to get plenty of playing time this season and it's reasonable to believe Barcelona will make it to a CL semifinal at the very least. It's quite a big step down to any club, apart from Bayern, from Barcelona.