Is selling a plot of land, whose installments are being paid off by a man to an usurious company at an annual interest rate of 6%, and then the full price was paid early to the company, considered valid or does it involve usury? And what should be done if that is the case?

If the company's fixed installments are part of the price (an increase agreed upon at the time of the contract for delayed payment), then there is no harm in purchasing the land, due to the validity of the initial contract between the seller and the company. The company's demand from the original seller for part of the price, or its referral of you to the company for the remaining land price, does not affect the ruling.