Engineer vs. Doctor

An Engineer is having trouble finding a job so he opens a medical clinic with a sign saying:
"Get your treatment for $500. If it's not successful we'll give you $1000!"

A doctor thinks this is a good chance to earn some cash, so he rocks up and says,
"I've lost my sense of taste."

The engineer replies, "Nurse, please place 3 drops from box 22 in the patients mouth."

The doctor tastes the drops and immediately screams, "This is petrol!"

"Congratulations!" says the engineer. "You've got your taste back. That'll be $500."

Annoyed, the doctor returns a couple of days later to attempt to recover his money and says, 
"I've lost my memory!"

The engineer has a bit of a think and replies, "Nurse, please put 3 drops from box 22 in the patients mouth."

"But that's petrol!" the doctor cries out again.
"Congratulations!" says the engineer. "You've clearly got your memory back, that'll be $500 please."

The doctor gets really pissed off, so he returns a few days later and says, 
"My eyesight has become extremely weak."

The engineer replies, "Well, I'm afraid I don't have any medicine for weak eyesight.
But here, take this $1000."

"But this is $500!" exclaims the doctor.

"Congratulations!" says the engineer. "You got your vision back! That'll be $500."