Galliani has already confirmed Milan's interest in the Argentine, who they failed to sign last year after his public falling out with Manchester City.
29-year-old Tevez is being strongly linked with a move away from City, as he just one-year left on his current deal.
Paris Saint Germain, Monaco and Juventus have all been linked, whilst Milan confirmed their interest earlier this month.
But Galliani admits their failure to sell players, such as Robinho, is hampering their attempts to bring players in.
"Tevez is a Manchester City player and we will not be doing anything unless someone else leaves first," Galliani told Sky Sports Italia.
"If another club wants to come forward for Tevez, then they are welcome to do so.
"If we do not sell one of our players, then we cannot make any other moves."
He admitted that Robinho's possible move back to Brazil with Santos was not moving on.
"Robinho, the negotiations with Santos are static. We are in talks, but have not reached any agreement on the figures," he said.
He also spoke about the futures of Kevin Prince Boateng and Stephan El Shaarawy - but he insisted there were no offers for either.
On El Shaarawy he said: "We never thought about sell him, there are no particular offers for him and at the moment everything is calm.
"There are no offers for Boateng. The player has not requested a move, nor have any other clubs made enquiries.
"I met with Monaco, but it was purely a courtesy visit. Nobody believes me!"
He did confirm that they have an interest in Sampdoria midfielder Andrea Poli, with M'Baye Niang linked in exchange
"Poli could be of interest to us, but only if someone else leaves. There are no Sampdoria requests for Niang," he said.
"We believe that the squad is competitive, so if we do not sell anyone then we won't bring anyone else in."