פירוש על קידושין 45:8

If a slave can be freed against his will through money, then why not also through a document accepted by others? Just as he is bought against his will with a document, so he can also be set free against his will with a document.
The answer is that these are different documents. And if you argue that the money used to acquire him is also different from the money used to purchase his freedom, we can answer that money is money. It is all stamped the same way.