Tosafos discusses whether hefker would be a viable option to free an eved ivri. Tosafos claims that hefker works by an eved k'nanin and should also work for an eved ivri. However, according to the opinion that hefker by an eved k'nani would not be sufficient to permit him to marry a jewish girl, there is a machlokes by an eved ivri if hefker alone would be enough. The Ritva holds that just as an eved k'nani requires a shtar to remove the issar aspect that forbids him to a jewish girl, similarly an eved ivri requires a shtar to forbid him to marry a shifcha k'nanis. However, Tosafos clearly holds that even though a shtar would be necessary for an eved k'nani, hefker would work completely for an eved ivri and no shtar would be necessary. The question is, how can hefker change his status and forbid the eved ivri to a shifcha k'nanis? R' Moshe (dibros, heara 5) explains that by an eved k'nani there are 2 seperate kinyanim, one is monetary that binds him to work and the other is a kinyan for the purpose of issur that forbids him to a bas yisroel. Therefore he is not permitted to a bas yisroel through hefker, since it does not remove the seperate kinyan for issur. However, an eved ivri is the same as any other jew. The right to marry a shifcha k'nanis is not a seperate kinyan in him that would change his status of being a regular jew. Rather, it is an outgrowth of being owned by someone else that his master is entitled to provide him with a shifcha. It is literally part of the master monetary entitlement that allows him to work with the eved by day and night as the gemara says 15a. Therefore, as soon as the ownership is removed using hefker, the eved ivri is automatically assur to a shifcha. R' Moshe is so convinced that Tosafos approach makes sense that he is troubled with the Ritva.