Civil law systems assume that an exchange of promises or a concordance of wills is the appropriate basis, not the exchange of precious rights. So if A promises to give a book to B and B accepts the offer without giving anything in return, B would have a remedy against the book and A could not change his mind to give it B as a gift. However, in common law systems, the concept of Culpa à Contrahendo, a form of estoppel, is increasingly used to create obligations in pre-contractual negotiations.   Estoppel is a just doctrine that provides for the creation of legal obligations where one party has given assurance to another party and the other has relied on insurance to its detriment. Generally speaking, the courts do not ask whether the agreement between two parties was monetarily fair, except that each party passed on a legal obligation or obligation to the other party. In other words, if you offered to sell your bike to your neighbor and you asked for your collection of antique cigar boxes in exchange and your neighbor agrees to pay that amount (i.e. give you her collection of cigar boxes for the bike), it doesn`t matter if the deal may seem unfair to some. You made an offer for the bike, your neighbour accepted it for a fee, and you both intended to make that deal, and you are both competent for that; So this is a viable treaty.