When you return an item, you lose the associated credit you originally earned by buying the item in the first place.
Sound kind of confusing? Let's take an example: let's say you had previously spent $50 towards a 'spend $100, earn 500 Golden Beans' promotion, and you decide to buy a $20 item, which bumps you up to $70. If you decide to return that item, your progress would also go back down to $50 – it's just like you hadn't bought the item in the first place.