Is it considered usury (riba) for a partner to sell his share of the land to his fellow partner at today's price, after he had registered it in his name based on a loan with an agreed-upon gift?

A loan with a stipulated increase is forbidden usury (riba), and this prohibition is not negated by calling the increase a "gift" or by the payer's consent. The sale that occurred is valid, and you are now the owner of a quarter of the land and have full discretion to sell your share or not. If you sell it, you are free to sell it for whatever price you wish, whether it is more, less, or equal to the loan amount, and whether it is at the market price at that time or not. You can sell it now or later. You are not obligated to sell it, nor are you obligated to sell it at a specific price to avoid falling into usury. If you wish to sell it to her, then sell your share for the agreed-upon price; it is not a condition that it be at today's price. Reducing the price out of consideration for the relationship between you is a good thing, and the Hadith of the Prophet, peace and blessings be upon him, applies to you: "May Allah have mercy on a servant who is lenient when he sells," and his saying: "Allah, the Mighty and Majestic, admitted into Paradise a man who was lenient when buying, selling, judging, and demanding repayment."