I talked with someone to sell him my car and we agreed on the amount, and he will give me the money after three days. After one day someone came and offered me more money than the first one. Do I have to sell to the first one or can I sell my car to the second one who will pay more money.
Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Your agreement with the first person can be either a promise or a contract. So, if you promised that the transaction will be concluded after three days this is a promise to sell and fulfilling one's promise is desirable according to the majority of the scholars and the Maliki believe that it is obligatory to fulfill it if any harm could be incurred from the opposite.
But if the agreement was already concluded whereby the car was sold and what is being awaited is the payment of the amount due, then this is a contract and the transaction was completed and it is unlawful for you to sell it to somebody else as it is no longer yours.
Allah knows best.