Is the amount offered by my colleague in exchange for him reserving an apartment for me permissible or impermissible, knowing that I do not want the apartment and cannot afford the down payment and installments?

If the condition for eligibility to reside in these apartments is not met by your friend, then it is not permissible for you to register them in your name on his behalf so that he may take what is not rightfully his. If this action is impermissible, then taking payment for it is even more impermissible.