Is it permissible for an employee, to whom the employer has allocated 75 piasters daily for his food, to take goods from the shop worth this amount for his family, or to buy food with it from outside the shop?

It appears that the permission granted allows you to own 75 (units/items), so you may take their value from the shop and dispose of it as you wish, unless you know that the permission is restricted to personal consumption within the shop. In that case, you are not permitted to exceed what was authorized. As for taking the value in cash, it is not permissible, because permission for materials is not permission for money. It is advisable to clarify the matter with the shop owner.