Is it permissible to return a garment that was purchased for one hundred Riyals in order to buy it again at a reduced price of seventy Riyals, knowing that the return period is still valid?

It is permissible for you to return the garment and reclaim what you paid, as long as the store's policy allows it within three days. This falls under the category of the permissible "khiyar al-shart" (stipulated option) according to scholars. However, if you wore the garment for reasons other than trying it on, then the use of the sold item nullifies the stipulated option according to the majority of scholars, because it indicates acceptance and confirmation of the sale, unless the use was for the purpose of testing and trying it out.