Q: I ordered a new bed set, and noticed at night that one of the mattresses was damaged. Can I sleep on it meanwhile until the mattress is replaced?
A: The Shulchan Aruch writes that defective merchandise can be returned, provided that you did not use the item after noticing the defect. However, if you use it afterwards, this indicates that you accept the defect and wish to uphold the sale, and you thereby forego your right to return the mattress (C.M. 232:3).
Many authorities maintain that even if you were unaware of this halacha and thought that you are allowed to use the item meanwhile, you still forfeit your right for redress (Pischei Teshuva 232:1.) Of course, the seller may elect to replace the mattress anyway.
If you notified the seller of the defect before using the mattress, some authorities maintain that you do not forfeit your right for redress, since you clearly did not accept the defect (Ibid.). Even so, since the sale is void, you have no right to use the mattress without permission from the seller, unless it is absolutely clear that he does not mind or that he will simply discard it.