Is it permissible to engage in a transaction with an Islamic bank that requires it to purchase a plot of land and then sell it to me in installments with a profit margin, and to mortgage another property whose value is greater than the price of the land, or to deposit an amount equivalent to this margin?

There is no objection to the bank purchasing the land and then selling it to you in installments with a profit agreed upon by both parties. This is a permissible deferred Murabaha (cost-plus-profit) sale, subject to its Sharia-compliant conditions. The most important of these conditions is that the bank must own the land and it must enter into its possession before selling it. The bank is also entitled to request a mortgage as security for its debt.