Hi,
I think I have found a solution.
You should send your friend the box unlocked, and inside the box there should be one of your locks. Your friends introduces one of his locks inside the box, and uses the lock he has received (your lock) to lock the box. Then he sends the locked box to you. When you receive the box, as you have the correct key, you can unlock the box. Then you should introduce the valuable object inside the box, and then lock it with your friends lock. When your friend receives the box he will be able to unlock it as you have used his lock
I hope it is clear enough :?:
JC