Is it permissible to deal with debt repayment using a plot of land, with an agreement to sell it and return the surplus or claim the deficit, and does this involve any suspicion of usury (riba)?

The debtor is obligated to repay what is due from him. If he borrowed money from you, he must return an equivalent amount. If he is unable to repay at the time or if you agree to accept an alternative, such as land or a house, there is no objection. It is permissible to accept food and goods in place of a loan. What was offered to you, namely appointing you as an agent to sell the debtor's land and then taking your debt from its value, is permissible and has no relation to usury.