Is it permissible to pay commissions or give gifts to employees of a client (contracting companies) in order to secure a tender or project for one's company, knowing that the client is unaware of this, and the quality among the companies is similar? And is working in sales in this manner permissible?

It is not permissible to pay commissions or gifts to employees in exchange for awarding a tender or project to a specific company, whether the amount is fixed or a percentage of the project. If the company pays bribes to obtain the tender, and your work assists in that, then it is not permissible for you to continue in it, and you must seek another job that does not involve anything unlawful or assisting in it. However, if your work at the company has no connection to unlawful activities, then there is no harm in continuing in it.