The American defender has been the subject of transfer speculation all summer, with both Blackburn Rovers and Cardiff City amongst the clubs said to be interested. However, he is in the last year of his contract and as a result could walk away from The Den for nothing. So The Lions are hoping to secure his services for at least another year.
"We are waiting over Zak. July is our deadline on that," Chairman John Berylson told the Southwark News. "We have tried to come up with a contract that would allow Zak to leave if we failed to win promotion.
"I don't want to clip his wings, but he is an important asset to us. We won't give him away, but we would agree that he could move on to further his career."