Is it permissible, from a Sharia perspective, to reduce the agreed-upon rent for an apartment with the tenant, such that the contract shows the full value while he retains the difference, and if that is not permissible, would I be sinful if I agreed to his request?

This person is a trusted agent by his company for the rental contract, and he must work for the benefit of his company. So, if he obtains a discount, it is for his company, not for him. If he takes something for himself without the company's permission, this is fraud, betrayal of money, and unlawful consumption of wealth. It is not permissible to agree with him or assist him in that, and whoever agrees with him has shared in the sin.