EXOD480 It is a negative commandment not to covet (desire) anything belonging to one's fellow-man as Scripture says, You shall not covet, etc. [this verse]. Now, coveting denotes that a person invest effort to put his thought into action; he sends many friends to the fellow, and importunes him, until he takes it [the object he desires] from him. Even if he has given him a great price for it, he thus violates it [the commandment]. This often occurs when a son-in-law pressures his father-in-law before the wedding that he should give him this-and-that object, which they did not stipulate when the t'na'im (conditions of the marriage) were written. Even if his father-in-law fulfills his demand, the son-in-law nevertheless violates his probation not to covet it, etc. (and see Rabad on Rambam, Yad, hilchot g'zelah, i).