Tennis Ball Problems (How do I get my dog to learn to "give"?)

My girlfriend and I recently rescued a 2 yr neutered male from the county pound.  It's only been a few days but he has displayed a very friendly, calm, and even temperment.  He is great on a leash on walks and runs.  He generally listens to us very well and has not tried to really challenge us.  However, he does seem to like tennis balls A LOT.  He has no problems letting go of other toys but will not give up on a tennis ball unless I bribe him with a treat.  Once I take it away and put it up he might stare at it for a few seconds but will move on without incident.  I don't want him to be that protective of anything.  I don't want a friend or family member coming over and touching something that causes an issue.  He does not growl or show obivous signs of aggression but I don't want it to escalate to that.  Any advice would be much appreciated. I am a first time pit owner and want to be as responsible as I can.  Judging by the comments and looks I received when I told people I was rescuing a pit, the last thing I want to do is to have to take him back because of something I could have prevented. Should I just not let him play with tennis balls?  Should I continue to bribe him with treats?  Am I over reacting to nothing?