Alright. From what I'm gathering from everyone is to let him go. Now, we(him and I) just finished discussing the matter. I didn't incriminate him or anything but simply asked his side of the story.
First, I failed to mention that he's a great tech and we work quite well one with another. He can handle most of what I give him. Now, after talking with him(after he finally remembered), he says he wanted the client taken care of. The store didn't have the RAM in stock so he went outside to check his car to see if he had any himself. He did and installed it for the client. No work order filled out...no documentation...$50 cash in the tech's pocket. The client calls me this morning with a message of I'm having start-up problems and he doesn't know if it's the RAM.
My Problem is...the client isn't going to mess with the tech...he's coming after the company. Now, the client is under control and will be in shortly so that I can resolve his issue but...you know.
SIDE NOTE:
The tech has given me various parts to complete several client's repairs(of which I asked how much do I owe him) but didn't charge me for them. He was looking at it(he said) like, "this was my RAM." I advised him of the potential dangers that could be ahead(you know how clients can go crazy with lies) and he understood. I sent him home for the day. I'm thinking about this seriously because he has caused more increase than decrease.
Off of this post, any more suggestions are still appreciated and thanks to the previous ones.