Adesie Slingback Sandal (Women)

I wanted some red suede heels to go with a red suede belt I recently purchased. The color was a perfect match. When I tried the shoe on however the curving strap that goes across the top of the foot is non-functional (doesn't secure the foot in any way), but I figured it would be similiar to wearing a mule since the toe strap was pretty snug. But when trying to walk in the shoe, the back of the strap slips under my heel whenever my heel lifts up from the shoe. There is not a way to adjust the tightness of the strap. I am taking them to a cobbler to see about adding a buckle to the strap and depending on the cost, will decide whether to return them or not.