Question: May Allah be good to you, oh possessor of virtue, this questioner says:
I heard that you said that the open buffet is haram? Is this correct and what is the reason?
Answer: This speech I did not say, this is a lie. I was asked about entering a buffet and a person eats what he wills and the price is fixed. You pay this price, fifty riyals, and eats whatever he wants without restriction. This is unknown. What he eats is unknown. It is required in buying and selling that the sale is known. I said this obscurity is not allowed. Yes.
If someone restricts what he will eat and he says I will eat such and such, there is no opposition. However, if it said to him: eat whatever you will and you only pay five riyal or ten riyal, then this is unknown. It is not allowed.
[Taken from: https://www.youtube.com/watch?v=xLJlSkIHNt4 ]
Faisal ibn Abdul Qaadir ibn Hassan