Corrupt Courier Service - Puzzle

Two friends Sachin & Rahul are living in two different towns. Sachin wanted to send few diamonds to Rahul but there is big problem. Both Sachin & Rahul have lot's of boxes which can be locked with locks & keys. Both have multiple locks. There is only 1 courier service which can ship those diamonds & there is no other way. Even worse is that workers at that courier service have bad habit of stealing the goods from packages. But one good thing is that they don't take efforts to break locks if boxes being shipped are locked. 

How Sachin should send diamonds to Rahul?

This is what Sachin should do? 

Dealing With Corrupt Courier Service

What was the situation? 

What should Sachin do is that send diamonds in box locked with his own lock & key. On receiving that box, Rahul should put his own lock on the box without disturbing lock placed by Sachin. With 2 locks, he should send that box again to Sachin. Now, Sachin should remove his lock with his key & keep lock placed by Rahul intact. Finally, he need to send back the box with Rahul's lock on it.

Now Rahul can unlock the box using key that he has to get diamonds.

In this way, in every journey there would be always lock (or 2 locks in journey of box from Rahul to Sachin) on the box. So workers won't get any chance to steal those diamonds.