In the absence of special governmental provisions,[1] the halacha is that after making a kinyan[2] on a sales item, a buyer cannot back out of the deal unless either: 1) The object was seriously blemished; 2) It was very overpriced; 3) A condition was made to allow it. However, we will work under the assumption (whose guidelines are beyond our present scope) that the ordinance that allows no-fault return is indeed binding. That ordinance was certainly not instituted to help people who are acting with intentions such as yours. Furthermore, even assuming that the law would apply to this case, you seem laudably aware that this does not mean that you are morally and halachically permitted to buy the dress with the intention to gain from it and then return it.
While we are not experts on this ordinance, our research indicates that it includes relevant limitations. For one, the consumer can return the item only if he has not used it. It is a good question whether handling a dress minimally in order to determine its cut is considered using it. We would assume that a use is a use, even if it is not a standard one and it does not wear out the dress.[3] Thus, if you hide your “use” of the dress, this would be misapplying the law. Another provision of the law is that the seller can demand, as a charge for returning, either 5% of the sales price or 100 shekels, whichever is lower. We will see that this provision may actually help you from a moral standpoint, but first we will look at the halachot of ona’at devarim (non-physical abuse), which are relevant to your question.
It is forbidden for one to ask a merchant the price of a sales item if he has no intention of buying it.[4] While some describe the classical problematic case as one in which the supposed buyer intends to upset the seller,[5] others refer to the psychological damage caused to the seller through the disappointment of losing a sale, irrespective of the buyer’s intentions. The Meiri[6] notes the possibility that the discussion of price may take away from others’ interest to buy the item at that price, and he writes that even if no one else is present, the discussion still causes the seller pain and toil. Although any negotiations with a proprietor can lead to disappointment, this is justified in a normal process of commerce (i.e., there is some chance he will buy). (Indeed, one who is overly sensitive should not be a storeowner.) However, when the proprietor has nothing to gain, it is forbidden, according to the second approach, to engage him for no reason.
It is not clear to what extent a salesperson who is an employee would be overly put out by the effort exerted to make the sale and/or upset by its return, although we cannot rule that out. In any case, there are a few potential scenarios of loss for the owner when you buy the dress without intention to keep it. By occupying the salesperson, you may discourage others from buying or prevent her from doing something else of value; while the dress is out of the store, it cannot be sold; and handling the dress may take away from its freshness, etc. While such concerns are not very strong, they may be enough to qualify as a violation of the halacha not to feign interest in buying.
On the other hand, if indeed you will have to pay, albeit modestly, for returning the dress (or if you will volunteer to do so), it stands to reason that this compensates for the small concerns and logically overcomes the problem of ona’at devarim. However, this does not solve the problem that the ordinance does not allow return after “usage.” In any case, we would urge you, if it seems possible (depending on the owner or worker’s personality), to be open and honest on the matter – request permission to do what you want for a modest, agreed-upon price.