What is the ruling on a hospital taking five hundred Riyals for twenty medical examinations over the course of a year, with no refund for the remaining amount if the examinations are not completed?

If the two parties agree on a specific number of medical examinations, and one of them misses some of them due to laziness or not being ill, then this is considered negligence on their part regarding their right. There is no harm in their right to these examinations being forfeited because it is known.