Is the testimony of a man's sons or parents accepted regarding his authorization (Wikalah)?

Primary text

The testimony of a man's sons or parents establishing his authorization is not accepted because they seek to establish a right of disposal for him, and a person does not establish a right for himself through the testimony of his son or father. Similarly, the testimony of the principal's (Muwakkil's) sons or parents regarding the agency is not accepted. The reasoning is that this testimony establishes an agent acting on behalf of their father (the principal), and this differs from testimony concerning a confession (*Iqrar*), as a confession is purely testimony against the confessor.