1. Question: How can Tosfos (Kiddushin 16a “Leima Ley”) say that a Jewish slave can go free by his master just declaring him ownerless just like a non-Jewish slave?
A. Shmuel holds a non-Jewish slave goes free even by his master just declaring him ownerless. Even according to the opinion that the slave needs a document of freedom that is only to remove his prohibition of marrying a Jewish girl but a Jewish slave can be married to a Jewish girl.
B. The Pnei Yehoshua’s question on Tosfos is that the way a declaration of ownerless works is that anyone whether rich or poor, man, woman, etc. is able to acquire the ownerless object. Potentially by the non-Jewish slave anyone can really pick him up once ownerless, but practically speaking he acquires himself for freedom, but potentially if he did not want to do so then he can become slave of someone else. However that does not work by a Jewish slave, once he is free, automatically only he has rights to himself, so he never could potentially be owned by someone else and therefore the rules of hefker, making things ownerless should not work by a Jewish slave?
C. The Torah doesn’t allow the laws of acquisition, zechiah, to work on a Jewish slave. The temporary owner cannot even sell him to another master.
Answer: Granted the Torah just doesn’t allow him to be acquired by anyone but that’s just a technicality, the slave himself and the owner potentially had in mind for anyone to take him when the owner declares him ownerless so the laws of hefker can work. Only if the owner limits who is allowed to take what he declares ownerless will the laws of hefker not work.