The legal principle "A person who has attained majority can enter into a contract" means that a person who has reached the age of majority, which is usually 18 years, is legally capable of entering into a contract. Such a person has the capacity to understand the terms and consequences of the agreement and is legally bound to fulfill the obligations arising from the contract.

In the given factual situation, a minor has entered into an agreement to sell his house to B and received Rs. 20,000 as an advance. However, the minor later withdrew from the agreement, stating that he is a minor. According to the Indian Contract Act, 1872, a minor is not competent to enter into a contract. Therefore, the minor's agreement with B is void ab initio, which means that it is treated as if it never existed.

Since the minor's agreement is void, he is not legally bound to fulfill any obligations arising from the contract, including returning the advance received from B. However, it is also important to note that B may have a legal claim against the minor's legal guardian or parent for any losses incurred as a result of the minor's actions.