This is a key command to any game of fetch . . . unless you want to be doing all the retrieving yourself. In addition to giving the verbal command, place your hand at doggie eye level with palm facing the pooch — it gives you the perfect placement to then receive the item as he learns to let it go right in your hand.
Remember, to teach a dog these hand signals, begin by using the word and the gesture and then alternate training until the pooch can do as you desire without you even opening your mouth!