Paying for medical examinations in advance
Question

                The head of a hospital is saying to the hospital’s clients: pay five hundred riyals for twenty medical exams in one year, whether for cases of sickness or for regular check-ups throughout the year. If a person uses all twenty exams, he has got what he paid for, but if he does not use them because he did not come for check-ups or did not get sick, he will not get any of his money back.
Praise be to Allah.We put this question to Shaykh Muhammad ibn Saalih
    al-‘Uthaymeen, may Allaah preserve him, and he answered as follows:
Have they agreed on a specific number?

Yes.

Then it is OK.

Question:
Even if he does not come for all twenty appointments, whether because
    he doesn’t get sick or he is lazy and doesn’t come for check-ups, so the five
    hundred riyals are gone?
Answer:
That means he has forfeited his right.
Question:
Yes, so if he gives up his right, for example, that’s OK?
Answer:
It is OK so long as what he has given up is a known amount. And Allâh
    knows best.