Wouldn't happen but could hypothetically regardless of FFP restrictions. It would need Barça to play ball though, A) loan the player with option to buy (contracts could be done in advance) & initial payment upfront B) buy on cheap to compensate for restrictions with huge inflated appearance bonus to be paid seasons end. Both options viable (as I said unlikely though).
With regards to wages, bonus' don't count so £100k pw with added bonus' totalling the difference would see that sanction scuppered too.
So in theory it's possible, in reality not a chance we'll see Messi join City anytime soon ;-)