Transfers are usually written down over the length of the contract. So if he signs a 5 year contract, 250m would go into the books with 50m for each of the following 5 years. Add around 20m in wages and we're talking about 70-75m Euro for 5 consecutive years. I'd say that United, Bayern and Real could afford it, maybe they need to get a bit creative with the accounting, but that wouldn't be anything new. It's not impossible. There's also a chance that Addidas would help to make it happen, so that the Adidas posterboy plays for one of the big Addidas clubs.Could a club like United/Bayern sign him regardless and just accept failing FFP for one year? You'd get a fine and transfer restrictions at worst I presume. Balance that alongside having Lionel Messi lining up for you week in week out and I don't think it's a difficult choice. Obviously that's a huge oversimplification but you see the idea.
Still, it's a huge huge risk. It would definitely limit further transfers in the upcoming years and in case Messi suffers an injury or never fulfills the expectations, it could cause a lot of problems. I highly doubt he'll leave, but if he really wants to, the big clubs can make it happen.