Is the owner entitled to sell the land to another person after reaching an agreement with me and taking a down payment?

What the questioner paid to the landowner is a earnest money deposit, not a down payment. A down payment is paid with the contract and becomes part of the price if the customer chooses to buy, otherwise it goes to the seller. As for the earnest money deposit, it is an amount paid to the agent to confirm the seriousness of the principal in requesting the commodity. If the principal changes their mind, the actual damage is compensated from this amount and the remainder is returned. If it does not cover the damage, the agent can claim the remaining loss from the principal. The landowner is not allowed to sell it to someone else unless you permit him, due to the prohibition of bidding over a bid. However, if he sells it, the sale is valid according to the majority of scholars, though sinful, and he must return the earnest money deposit you paid.