Have I committed a sin by helping an agent who owes me money obtain a usurious loan by writing a letter to the bank stating that he is an employee of mine, and what must I do to purify myself of this sin?

Agency for a fee is permissible in Islamic law. A fee that is a percentage of the profit is also permissible, though there is a difference of opinion regarding this. The agent is required to collect the price, but he does not guarantee the price unless his transgression or negligence is proven, because he is a trustee. The condition that obliges the agent to guarantee the price in all circumstances is void, and whatever was taken from him under this guarantee must be returned. It is not permissible to force him to borrow with usury, because guaranteeing a person to borrow with usury is unlawful.