Use of equipment with a deposit based on return is a hassle. You have to track the deposit money separate from the cost of the bees, you have to track the time frames in which the deposits are refundable, you have to track who has what equipment (in my experience, that equipment rarely comes back in a timely fashion). It can be done, yes, but in the height of the season, it's a lot of tracking. And if someone's helping you (usually a family member?) then it might be hard to keep all those options straight.
Just use a cardboard nuc and call it good. They aren't that expensive and you don't have the hassle of different pricing based on whether or not someone brings a box. Also, it's a good way to get an extra box into a customer's inventory. I've never had anyone say, "It was a hassle storing that box." But I have had many people say, "I was really glad I had that nuc when my hive swarmed!"
If they really want to return it and ask about doing so, take Mike's advice and pay back 1/2 the cost of the nuc. But don't make it a deposit. That way, if it doesn't come back, you haven't lost anything because you already factored it in to the overall price of your product.
Oh, and congratulations on getting the green light!