Is it permissible to take out a loan to buy a plot of land, through an Islamic bank with murabaha, after concluding a promise-to-sell contract worth 20 percent with the original owner?

The meaning of your statement: "a sale promise contract worth 20 percent" is not entirely clear to us.

We have previously explained the necessity of severing every contractual relationship between the client and the original seller in a Murabaha contract, if the bank is to purchase the commodity in full and sell it to the client on a Murabaha basis, so that the transaction does not devolve into an interest-based loan.