The only clubs that could really buy him are City, PSG, Chelsea (?) and Bayern. Out of those i think only PSG would be potential suitors, because if they sell Cavani and Lavezzi, which is likely to happen this summer, then they would be able to pay for Messi without breaking FFP rules whilst clearing out their XI for Messi to fit in.
Either way, Messi will get his way, Barca will do what he wants and he wont move.