Is it permissible for my brother to respond to the request of his colleagues at the bank to take the Murabaha amount designated for a specific commodity and use it to purchase something else (such as an apartment), given that they cannot submit a direct request to purchase an apartment?

Your brother must refrain from this transaction because it is a usurious loan, as colleagues receive one hundred thousand and owe more than that in their liabilities due to the waiting period, and this is usury. Moreover, this transaction contradicts the terms of the agency that the bank granted to your brother, for the bank authorized him to purchase a specific commodity with this amount, and he is not permitted to violate that. Ibn Qudamah said: "The agent does not own any disposal except what his principal's permission necessitates, whether by explicit statement or by custom; because his action is by permission, it is confined to what he was permitted to do."