Allow only one object in drop target when it's not a question

I'm trying to create a drag and drop interaction that is not a free-form question. When the user places a number in the target a layer with information about that object is revealed. I want to only allow one object in the target at a time, so if the user drops a new object on top the original object will return to it's starting position.

I've seen recommendations to do this by restarting the slide using triggers, but that means you can't have any audio on the base slide and I want to have instructions.  I also want to make progress past this slide conditional on having looked at all 5 layers. So if the slide keeps resetting to move the objects back to their original position it won't look like you've made any progress visiting the layers as each layer would be a fresh start on the slide.

Thanks for any help!