We also send my step-daguhter to mom in whatever she arrived in. If for some reason we can't, we send her in something we don't mind never seeing again... we usually had a few hand-me-downs on hand for just that reason. Her mom was actually pretty good about getting clothes back to us, but we never sent something if we absolutely had to have it back.
It started because mom wanted her clothes back and we didn't have many to spare... but it actually became a part of her back-and-forth transition... when she was going back and forth every 2-3 days when she was younger, she could sometimes feel like she didn't know where she was going to be from one day to the next... but we would lay out her clothes the night before, and if she saw "mom's" clothes, she new the next day was a transition day... it actually seemed helpful to her.
Now it has become habit, and it's just part of life. Mom tends to send her on the airplane in comfy, layered travelling clothes, so it's easy to put her in the same thing to travel back the other direction.
When there was a problem with weather-appropriateness, or something stained or too small, we'd dress her in something that we didn't mind not getting back, and send the other clothing item back in her bag. If we were picking her up and planning to go somewhere right away, we made sure to have appropriate clothing with us-- like if we were going to the park after school, we made sure to bring sneakers and play clothes in case she was in a fancy dress and dressy shoes.
Parenting four little monkeys (11, 8, 6, and 4) with the love of my life. Making it up as I go.