Is it permissible for the treasurer to take the slight surplus from the company's income, knowing that he covers any deficit from his own money?

It is not permissible for a treasurer to take surplus money, because it does not belong to him; rather, it belongs to the institution or its clients. As for a deficit in money, it is not necessary to guarantee it unless negligence or dereliction in accounting or preservation is proven, for a trustee is only liable for transgression or dereliction.