(Ibid.) "If he lacks it, he is to be sold for his theft": I might think, forever; it is, therefore, written (Ibid. 21:2) "Six years shall he serve, etc.", whence we are apprised that he serves for six years and goes out free on the seventh.
This Legal Connection Reveals Something Important About How the Torah
Mekhilta Tractate Nezikin 13:9