Is it permissible for a financing company to pay the remaining amount for an apartment to the first company in exchange for me repaying them in comfortable installments, and if that is not possible, is it permissible to purchase a commodity in installments and then sell it to complete the remaining amount?

It is permissible to purchase a car or shares with the intention of tawarruq (monetization) to obtain cash and use it to pay the price of the apartment. However, it is not permissible for the second company to pay the remaining debt you owe to the first company, because that is a loan with usurious interest.