Ruling on buying from “all-you-can-eat” buffets
Question

                What is the ruling on buying from open buffet restaurants, where the customer pays a set amount and eats an unknown amount of food that is not known to the buyer or the purchaser, rather he may eat until he is full? Is this a gharar (ambiguous) transaction?.
Praise be to Allah.
    Firstly: 

    Muslim (1513) narrated from Abu Hurayrah (may Allaah be 
    pleased with him) that the Prophet  (peace and blessings of Allaah be 
    upon him) forbade gharar (ambiguous) transactions). 

    Gharar in Arabic means a risk which  is not certain; it may 
    happen or not, such as selling fish in the water or birds in the air, 
    because the purchaser may or may not get it. 

    Al-Azhari said: Gharar transactions include any transactions 
    in which something is not known. 

    Mu’jam Maqaayees al-Lughah 
    (4/380-381); Lisaan al-‘Arab (6/317). 

    Al-Nawawi said in Sharh Muslim: 

    With regard to the prohibition on gharar transactions, this 
    is one of the important principles in the book of financial transactions, 
    and it includes many issues, such as selling something that is not there or 
    is unknown, or selling an animal that is still in the womb. All such 
    transactions are invalid because it is unnecessary ambiguity. 

    Some transactions may include some element of ambiguity out 
    of necessity, such as not knowing about the foundations of a house, or 
    selling a pregnant sheep or one with milk in its udders. These sales are 
    valid. Similarly the Muslims are unanimously agreed that it is permissible 
    to sell things in which there is slight ambiguity, such as selling a quilter 
    garment with its filling, but if one is buying the filling separately, that 
    is not permissible. And they agreed that it is permissible to rent out a 
    house, an animal or a garment etc for a month, even though the month may be 
    thirty days or it may be twenty-nine. And they agreed that it is permissible 
    to sell admission to bath-houses for a certain amount even though people 
    will differ in how much water they use and how long they stay. And they 
    agreed that it is permissible to drink from a waterskin in return for 
    payment even though it is not known how much is drunk and the habits of 
    those who drink vary. End quote. 

    It says in al-Mawsoo’ah al-Fiqhiyyah (31/151): 

    In order for ambiguity to have an effect, it should be great. 
    But if the ambiguity is slight, then it does not affect the transaction. 
    Al-Quraafi said: ambiguity and unknown matters – i.e., in selling – are of 
    three types: a large degree is forbidden according to consensus, such as 
    selling birds in the air; a small amount is permissible according to 
    consensus, such as the foundations of a house and cotton filling of a 
    quilted garment; there is a difference of opinion concerning a moderate 
    amount, is it regarded as the former or the latter? 

    Ibn Rushd al-Hafeed said: the fuqaha’ are agreed that a large 
    degree of ambiguity in transactions is not allowed, and that a little is 
    permissible. End quote. 

    Secondly: 

    What some restaurants do, offering an “all-you-can-eat” meal 
    for a fixed price, seems to fall under the heading of slight ambiguity which 
    does not affect the validity of the transaction. It is akin to what 
    al-Nawawi (may Allaah have mercy on him) mentioned about entering 
    bath-houses for a fixed price, even though it is not known how much water 
    will be used, or drinking from a waterskin even though it is not known how 
    much water will be drunk. 

    But if a person knows that he will eat an extraordinary 
    amount of food, then he should mention that to them, because then this is a 
    great ambiguity. 

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: 
    There are stores that sell food and say, “Pay twenty riyals and eat all you 
    want”. He said: It seems that this may be tolerated, because the size of an 
    average meal is known and this is something that is allowed by custom. But 
    if a person knows that he eats a lot, then he must tell the restaurant 
    owner, because people vary. End quote from al-Sharh al-Mumti’ 
    (4/322), Fajr Centre edition. 

    And Allaah knows best.