Is it permissible for a buyer to send another person to purchase land on his behalf if he knows that the seller will raise the price upon learning of the original buyer?

There is no objection to someone buying something for you without informing the seller that the purchase is not for him. There are two ways to do this: The first is for this person to act as your agent in the purchase, and intention suffices for this. The second is for him to buy it for himself and then sell it to you for the same price or more. The first method is preferable.