Is selling land that the questioner bought from his relative in installments, then selling it to a third party at a higher price before fully paying the relative, actually forbidden or merely doubtful?

What you asked about has two scenarios: The first is that the sale to the second buyer takes place after the completion of the purchase and possession between you and your relative. In this case, there is no blame upon you for selling the land at any price. The second is that the sale to the second buyer takes place before the completion of the purchase, or after the purchase but before possession. In this case, it is not permissible for you to sell the land until the sale and possession are complete, due to the prohibition of selling what one does not own and selling before taking possession, as stated in the Hadith (definition): "Do not sell what is not with you," and the Hadith prohibiting the sale of goods where they are bought until the merchants take possession of them and transport them to their own places.