What is the evidential requirement for an agent claiming to have paid a debt using funds provided by the principal?

Primary text

If an agent claims to have paid the principal's debt and delivered the funds to the creditor, the agent's statement is not accepted against the creditor without proof (bayyinah). This is because the agent is not considered a trustee (amin) in relation to the creditor. If the creditor swears (or if the agent provides proof), the creditor's debt is not discharged by payment to the agent; thus, the creditor retains the right to claim against the principal.