Viduka is now resting at home in Australia following his walk-out from Parkhead without playing a match. He said at a news conference yesterday that he was an emotional wreck after receiving a torrent of abuse from Zagreb's supporters.
Zagreb's spokesman Goran Bradic said: "The transfer fee was due last Thursday but we are prepared to wait another 10 days if necessary. If we do not receive the money by then we will have to turn to Fifa's and Uefa's lawyers to see what we can do."
Viduka insists he will return to Parkhead but claimed he had been unable to shop or eat out in Zagreb after being abused in the street. Bradic said: "Marko is a good guy, polite and honest, but maybe in some games for us he didn't try so hard. Once you get on the bad list of our fans it is almost impossible to win back their support.
"Our supporters are cruel. They don't think about people's emotions. For the past two years they have been taunting our president that he is a homosexual. They shout about your mother and your wife. But it's not all our supporters who act like this, maybe 5,000 out of 40,000. But if you are a public person who is highly paid then perhaps you have to put on a brave face."
Viduka walked out on Celtic nine days ago on the eve of his debut against Hearts at Tynecastle. He said: "I thought that once I had escaped to a new country with similar values to Australia that I would be able to once again excel at the game I live for and love. But that wasn't the case. So I decided to come back home - to recover, get my strength back - and then return to the only game I have ever wanted to play."
With the Premier League closing down on 3 January for three weeks, Viduka is unlikely to feature in Celtic's plans until February.