Quote from Mick McCarthy at the time...not sure if the sell on clause is still valid of course, but it would appear there was one.
"McCarthy said of the new arrival: “There’s no transfer fee – there’s a clause in it if we get promoted, a sell-on clause, but there wasn’t a fee."
TWTD (Independent Ipswich site) has this...
"Bialkowski joined the Blues from Notts County on a free transfer in the summer of 2014 - but with the deal including a sell-on clause entitling the Magpies to a percentage of any future fee"