Is it permissible for the husband to take the excess from currency exchange rates that the company does not take, knowing that the company obliges him to pay the difference from his own money if the market price falls below the specified price, and that the financial manager has authorized him to do so, in addition to the company shortchanging him on housing allowance?

There is no harm for an employee if the company obliges him to sell currencies at a specific price, and he is bound to adhere to it. If he sells for less, he guarantees the deficit due to violating his employer's order. If he sells for more, the increase belongs to the company, unless it permits him to take it. The company's failure to pay full housing allowance or its reduction of the salary does not justify encroaching upon the company's money.