Charles-Edouard Coridon's agent insists that Celtic are still interested in signing the Frenchman.
Charles-Edouard Coridon has impressed Celtic's players
The Lens midfielder, also interesting Everton, flew back to France after playing for Celtic in Tuesday's friendly against Tottenham Hotspur.
But Willie McKay said: "I spoke to Martin O'Neill this morning and negotiations are ongoing.
"He wants to see what he would cost in transfer fees, because he is still contracted to the club."
O'Neill had appeared lukewarm about Coridon's display in the 2-0 defeat of the English Premiership side and prospect of a £1.5m transfer.
But McKay said: "It's up to Celtic and Lens to come to an arrangement, but there won't be any loan deal because he has a year to go.
"Martin reckons Charles is a good player and Charles is hoping a decision will be made by Monday."
Coridon, who had impressed members of Celtic's squad during his few days in Glasgow, said: "I've enjoyed my time with Celtic and I want to thank the manager and the players.
"If Celtic decide not to take me then I have the opportunity to play in the English Premier League, but my first priority is to play for Celtic."
The 31-year-old was watched by Everton manager David Moyes against Spurs and O'Neill said: "He did okay.
"Obviously I will have to speak to him, but I don't know the exact position with his club.
"It's difficult to make an assessment after 90 minutes. Match fitness is another issue and he looked to be blowing quite hard at the end."
Coridon had been happy to go on trial as he recalled the experience of compatriot Eric Cantona, who tried out for Sheffield Wednesday before going on to greater success with Leeds United and Manchester United.
Playing in the Champions League with Celtic is a big attraction for the Martinique international.