הלכה על חולין 165:31

From the laws of the commandment is, for example, that which they, may their memory be blessed, said (Chullin 59a) that fins are what in the fish is like a type of wings with which to fly; and scales are that covering that clings to all of its body. And even if scales are only found on part of its body - and even it is only one - it is enough for it and it is fit (kosher). And they said in Tosefta Chullin 3:9, "And that is when it is under its jaw or under its tail or under its fins; but on the rest of its body, it is not enough for us with less than two." And everything in which scales are found - even one in the places mentioned - need not be checked [to ascertain] if it has fins; as it certainly has [them]. But if fins are found on it, it needs to be checked [to ascertain] if it has scales - as there are many impure fish that have fins. And any fish the nature of which is to have scales, even though they are not produced on it when it is still small until it grows up - such as the sultanit and the afian - behold it is permissible. And anything that has scales when it is still in the sea, even though it drops them at the time that it goes out from the sea - such as the akonas, the afonas, the bisifityas, the afnasetiyas and the tunny that is called biret in the vernacular - behold, it is fit (Chullin 66). As the Torah is only concerned with the species that it have scales and fins - since that is what is seemly for the nature of people, and from the reason that we wrote in the prohibition of foods according to the simple understanding.

And the law of how is the selling of consecrated properties or synagogues, and for which matter is their selling permitted (Arakhin 21b). And the law of buying deeds - that they require the handing over of the deed and also writing that makes explicit that he is selling him the deed and any lien that it has (Bava Batra 76a). And the law of what would be if one gives money to his fellow to buy him something and he buys it for himself with that money; and the law of three that gave money to one, and he went and he bought with part of the money, how do they divide it. And the laws of that which they said, at four periods in the year does money [itself actually] acquire beasts - and therefore they, may their memory be blessed, said in the chapter [entitled] Oto ve'et Beno (Chullin 83a) that we make a butcher slaughter [an animal once he received the buyers' money] against his will; and like the contextualization of Rabbi Eelai that Rabbi Yochanan said - which explained that [the Sages] stood their words upon the words of the Torah at these four times, and like Rabbi Yitschak said that Rabbi Yochanan said, "Money acquires from Torah writ." And what is the law (Ketuvot 76b) of one who sells a beast and it is found to be 'torn' (terminally ill and, so, unfit to eat). And the laws of a seller who says [the sale] is for a hundred and a buyer who says [it is] for fifty, and each one goes away, but he buys it afterwards undifferentiatedly. And the law of that which they said that a man may not sell what has not [yet] come to the world (Yevamot 93b). And the law of the owner of the sale; the law of a person's agent, and his partner in his buying and his selling regarding his loss and his profit. And the rest of the many details of the commandment are elucidated in the first chapter of Kiddushin and in the fourth, eighth and ninth chapter of [Bava] Metzia, and in the third, fourth, fifth, sixth and seventh chapter of [Bava] Batra. And a few of them are in scattered other places in the Gemara (see Tur, Choshen Mishpat).