The 22-year-old had been linked to the Serie A side as well as AC Milan, West Ham and Valencia before eventually joining Porto for €20 million this week.
And Willy Ndangi expressed his disappointment in the way Inter acted, having initially encouraged his son to join the San Siro club.
"I was very interested in his move to Inter, because it is a big club with an ambitious project," he told Journal du Dimanche. "We waited in vain for Roberto Mancini to speak to him, but for four days we did not have any sign of life from them.
"The way they acted makes me sick. They only woke up when they heard that there was interest from Porto. And their intermediaries also sent me a series of threatening text messages."
Ndangi also said an English agent tried to get involved to turn the player towards the Premier League.
"He had contact from England. Tottenham were watching him and West Ham played spoilsport. Willie McKay - an English agent - offered me €1m to take control. But let him go and spend that in the casino!
"He called Marseille president Vincent Labrune to say he was the new agent of Imbula. Labrune did not believe him, but can you imagine someone doing that?
"Anyway, Giannelli wanted a team who had qualified for the Champions League because he needs the boost to get into the plans of France coach Didier Deschamps ahead of Euro 2016."
Reports said that Ndangi was to receive €2m from the move which sees his son join Porto, but he insists he will not receive that much.
"If that were the case, I would be happy," he said. "But this is wrong! My commission varies between five and eight per cent of his annual salary."
Imbula is the latest in a number of star players Marseille have lost and Ndangi maintains that if the club was in a better financial position, he would have stayed in Ligue 1.
"Yes 100% he would have stayed. But Marseille told us they want to rely on the market value of Giannelli to sovle their money problems. Without that sale, the club would have had to part with four more players."