Is it permissible to inflate sales invoices for customers and return the excess to them, and is it obligatory to terminate the contract of an agent who pays bribes to attract business for the company?

It is not permissible for you to inflate the invoice value beyond the real price so that the broker takes the difference; for this is consuming people's wealth unlawfully and assisting in sin. If it is proven that your agent offered a bribe, it is not permissible for you to appoint him again later, because it is cooperation in sin, and the Prophet, peace and blessings be upon him, cursed the briber and the bribed.