Is the contracting company's dealing with the bank by depositing post-dated checks to obtain the full amount upfront considered usury that falls upon me, knowing that the apartment will be finished for me by installments?

There is no شرعًا (Islamically) any objection to concluding a contract for finishing an apartment in installments via monthly checks. It is permissible for the deferred value to be higher than the cash value, provided this is agreed upon from the outset. However, the contractor selling the checks to the bank for less than their value before their due date is محرم (forbidden) usury (riba). The accountable parties in this transaction are the contractor and the bank, and you should advise the contractor regarding the شرعي (Islamic legal) ruling on this matter.