How to make money as an engineer

An engineer who has been unemployed for a long time decides to open a medical practice.
He hangs a sign on the front door with the following inscription:

„For 500 EUR, I guarantee to cure your illness. If I fail, you will receive 1,000 EUR.“

A doctor by training thinks it would be easy to earn 1,000 EUR here and therefore visits the newly opened practice.

Doctor: „I've lost my sense of taste.“

Engineer: “Nurse, please bring me the medicine from the box labeled ’22‘ and administer three drops of it to our patient.“

The sister does as she is told.

Doctor: „Ugh, that's gasoline!“

Engineer: „Congratulations! You got your sense of taste back. That will be 500 EUR.“

The doctor is angry, pays the 500 EUR, and leaves the practice. After a few days, he returns.

Doctor: „I've lost my memory. I can't remember anything anymore.“

Engineer: “Nurse, please bring me the medicine from the box labeled ’22‘ and administer three drops of it to our patient.“

Doctor: „22? That's gas!“

Engineer: „Congratulations! You've regained your memory. That will be 500 EUR.“

The doctor pays through clenched teeth and leaves the practice. A few days later, he returns, determined to get his lost money back.

Doctor: „My eyesight has deteriorated significantly. I can only see outlines now!“

Engineer: „Well, unfortunately, I don't have a suitable medication for that. Therefore, as promised, you will receive the EUR 1,000.“

He gives him two 5 EUR notes.

Doctor: „Wait, that's only 10 Euros!“

Engineer: „Congratulations! You've regained your vision. That will be 500 EUR.“