Is it permissible for a mother to back out of her agreement with her son regarding the sale of land, and consequently, his right to the agreed-upon increase?

The statement of a mother to her son: "Sell this land for such-and-such [a price], and whatever exceeds that is yours," along with specifying a period for the sale, is considered a valid agency with a fee (ju'l) and for a specified duration. The son is entitled to the excess if he sells the land for more than the specified amount; otherwise, he receives nothing. This transaction falls under the category of ju'ala (a contract of reward or commission), which is a permissible and non-binding contract. However, if the mother retracts after the son has begun the work, she is obligated to pay him the wage of the like (ujrat al-mithl) for that work.