See Chidushim Ubiurim, end of Chulin.
Also, see the great “Shmittah Kemitzvasa” (at the end) against Heter Mechira. (I think it appears as “Kuntres al Issur Heter Mechira” on Otzar Hachochma).
Hyehudi.org
Reviving a Dead Religion Once Known as ‘Judaism’
