Is the increase that my husband, a used car salesman, takes on the agreed-upon price of a car with its owner permissible for him, knowing that this is common among merchants, even if he does not inform the car owner of this increase?

If the custom mentioned by the husband means that any amount exceeding the known price set by the car owner when placing it for sale with the showroom owner belongs to the showroom owner, then there is no harm in that. Some jurists have validated this, citing the action of Ibn Abbas, as it is a capital that grows through work, similar to Mudarabah (definition) (profit-sharing partnership). So, if the custom is prevalent in this regard, there is no objection, because what is customary is as if it were a stipulated condition.