Is it permissible for an employee of an Islamic bank, which offers "murabaha" financing for housing, to use this murabaha to acquire an apartment that he rents out, provided its owner agrees to receive the remaining installments, and can he reclaim the amount he paid in advance from the murabaha?

If the bank purchases the apartment with a valid sale contract and it enters into the bank's possession, and then the bank sells it to you with a valid sale contract, there is no objection to that. It is possible to agree with the apartment owner to terminate the lease contract and for you to reclaim what you paid before the sale to the bank, if the lease contract was valid. However, if it was a lease-to-own contract, it is invalid, and the apartment owner must return what you paid, and he is entitled to the fair market rent for the past period. Then, he can sell the apartment to the bank so that you may purchase it from the bank.