How do I transfer my Day of Defeat Source to my friend's acc?

I bought DoDS before I bought CSS, after I bought CSS i figured I have 2 cd keys that i used on one same acc. So how do i transfer one of them to my friend's acc? cuz when i tried, it said that its already in use.|||You can't. You need to give him your email and password. That's the only way he can use that CD-Key.