It is possible to check the squad numbers that the clubs offering you contracts have available - if you close the "offers" screen and go to league information and then team information, you can look there. It is annoying, but brilliantly my player managed to snag the number 9 shirt at Espanyol.
I have had a problem though - one of my other BAL players asked for number 32 at Torino, Tevez stylee, and I'd checked beforehand and number 32 was definitely available; however I got number 25. It could be because I was renewing my contract and had previously worn number 25, but I hope it doesn't automatically give you a lower number than you asked for if a lower one's available...