Is it permissible for a merchant to give a purchasing agent 5% of the sales value in cash and without the employer's knowledge, to ensure continued dealings, and what should the merchant do if this is not permissible Islamically, and is all his money then considered unlawful, and are his children sinful if they partake of this money?

The purchasing agent is a representative of the buyer, and he must work in the interest of his client to obtain the best prices and goods. It is not permissible for him to take a commission from the seller or to record an unreal price on invoices, because that constitutes deception. However, these practices do not make the merchant's money unlawful, nor do they affect his trade. There is no objection to his family and children benefiting from his money, but he must fear Allah and cease those forbidden actions.