Once your rabbit is picking something up, place your hand right underneath the object, so when they drop it, it falls into your hand. Mark or reward this a few times. Then, move your hand just ever so slightly away.
Did your rabbit move to place the object in your hand? If so, yay! Mark or reward. If not, you may have to go back to the first step a few more times. Otherwise, continue to move your hand farther and farther away, as long as your rabbit is following your hand to place the item in it.
Then, start to put the ball just out of reach of your bunny. Do they go get it? If so mark or reward. Repeat this a few times, gradually adding distance between the bunny and the ball. Now, place your hand close to them again to combine getting the ball and giving it back to you. Gradually increase distance.