"In the twelfth book I will include commandments regarding sale and acquisition. And I have called this book the Book of Property."

69. Sale - Mechirah

A sale or rental is not final until the proper form of acquisition has been followed. Real estate is acquired by exchange, by payment, by a bill of sale, or by taking possession. Movable property is acquired, once its price has been settled, by lifting it; by renting or buying land together with it; by exchange; by pulling it (or leading it, if it is an animal; this method applies only in certain types of domains); by turning it over to the buyer (if it is too heavy to lift or pull; this applies only off the seller's property); or by bringing it onto the buyer's property. Intangibles and things that are not yet in the seller's possession cannot be acquired; but bills can be acquired rabbinically by bill of sale and turning over, or with land.^a

A forced sale is valid unless the seller has annulled it in advance. A sale or gift can be contingent on conditions; these must be made in advance and must explicitly specify the alternatives ("if... and if not...") unless the sale is to be retroactive. In any case the conditions must be feasible, and in non-business matters (marriage, divorce, etc.) they must not be in conflict with the Torah. A promise to sell or give if certain conditions are met is not a binding sale unless it is registered in court; but it is not proper to go back on one's word.^b

It is forbidden for the buyer or seller or renter of real, movable property to deceive, as it says "And if you sell a sale to your friend or buy from your friend you shall not cheat each other".¹ If the deception is qualitative or amounts to more than a sixth of the price the sale can be annulled. It is also forbidden to insult anyone; and there are special prohibitions against cheating or insulting a proselyte, as it says "And you shall not cheat each other but shall fear your G-d",² and it says "You shall not cheat a stranger and you shall not oppress him".^3,c

In all matters of business local customs and civil laws can be followed.^d

Sources:
1. Lev. 25:14	a. 1:3,18; 2:5; 3:1,4,7-9; 4:1,3,11-12; 5:1,5,14; 6:11-12,14; 22:1,5,13
2. Lev. 25:17	b. 7:8; 10:1; 11:1-2,13; Zechiah u-Mattanah 3:6-8; Ishus 6:1-2, 9,13-17
3. Ex. 22:20; see Lev. 19;33, Ex. 23:9	c. 12:1,4; 13:8,17; 14:12,15; 17:1-2
	d. 26:8; 27:11; Zechiah u-Mattanah 1:15