FWIW, Fraser (show type) has no hesitation about handing over anything I ask him to retrieve for me. (I did post recently about him showing perhaps guarding/possessive behaviour toward me from greeting attempts by another dog.) He does it in frisbee/ball play and as I've got a bad back, I've been teaching him to pick up and give me things I drop like gloves, mail, cane etc. No problems whatsoever in getting him to hand over.
ETA: mind you, if it's a dead fish spine or some other tasty treat, different story.