Is testifying that the guarantees (checks and promissory notes) which companies take from sales representatives are an actual debt that the debtor received from the company and has not repaid considered void and unjust testimony, especially if some representatives claim that these guarantees are a prerequisite for employment?

This answer summarizes two situations for agents: The first is that they act as agents for the company in selling goods. In this case, they are not liable for the goods unless there is negligence or transgression on their part, and it is not permissible to take checks from them. The second situation is that they purchase goods from the company without paying the price and then sell them to others. In this case, there is no harm in taking checks or documents equivalent to the value of the goods to secure the debt, as checks are a permissible form of debt documentation.