Is the additional amount I will receive from the company, in exchange for not taking possession of the apartment and selling it at the current price, permissible or forbidden (halal or haram)?

The first solution is permissible if the construction costs differ significantly from the contract time with an exorbitant increase. In this case, the company has the right to demand a fair value for the price difference, and the matter should be referred to a judge or arbitrator in case of dispute.

The second solution is that if the two parties reconcile on selling the apartment at the current price so that the questioner benefits from the price difference and takes the profit for herself, there is no harm in that if both parties agree to it.