Riordan, who collected his first international cap, midweek against Ausrtia, was Hibs top scorer last season, with 23 goals and has scored 46 goals in
little over 2 seasons.
Cardiff have asked for the fee to be undisclosed, but with Hibs having turned down a bid of £1m from Cardiff just a few weeks ago, the new offer is thought to now satisfy the Easter Road board.
Hibs have now given Riordan permission to speak to the Welsh club.