All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
What is called "open buffet" in a way that a man pays a given amount of money and then he enters and eats all he can from whichever food he wishes, is an issue about which Shaykh Ibn ‘Uthaymeen was asked: “There are some restaurants that say: 'Pay 20 Riyals and eat as much as you can' what is your opinion on this issue?” He answered: “It appears that this is usually tolerated among people, as the size of an average meal is known, and this is something that is tolerated according to the customs. However, if a person knows that he eats a lot (an extraordinary amount of food), then he must mention this to the owner of the restaurant because people vary in this regard.”
Allaah Knows best.