A Championship club has made the bid, which if accepted would be a new record transfer fee received by the Robins.
Chairman Paul Baker said: "We do not want to accept the offer but it is such that if Kay really wanted to go we would have little option.
"He has another year on his contract and we would like the chance to speak to him and try to keep him here.
"There is also a sell-on clause with Forest Green Rovers, so we would not get the full amount."