بسم الله الرحمن الرحيم
When the investor occupied the plot which he bought then he became its owner and it shall be lawful for you to buy it on whatsoever price. However, the price should be fixed no matter if the payment is made in instalments. For example, you say that you bought the plot in ten lakh rupees and the owner of the plot accepts it and both of you fix the payment of ten lakh rupees in instalments in a way that you will pay one lakh rupees on the first day of each month and thus pay the whole amount in ten months, it is correct as per the Shariah.
However, if you settled the dealing in some other ways then write it clearly and submit the query and know the ruling.
Allah knows Best!
Darul Uloom Deoband