The South Africa defender is scheduled to join the English Premier League side in January but Sibiya has warned that United's terms have yet to be met.
"This is not a foregone conclusion and Tottenham will have to pitch a new and better offer during the next transfer window. Negotiations will open then," Sibiya told the Sowetan.
Sibiya declined to reveal Khumalo's price tag but insists Matsatsantsa have the 23-year-old's best interests at heart.
"People have very short memories when it comes to these kind of things. I can assure you that Bongani will look back and thank us," he said.
"I am of the opinion that the player has a lot of potential and if he is patient and concentrates on cementing his place in the national team he will go far.
"I feel sure that in a few months Bongani's stature will have risen and he will secure himself an even better overseas move."