Kautilya's Arthashastra 3.13.25

3.13.25

dāsaṃ dāsīṃ vā niṣkrīya punar vikrayādhānaṃ nayato dvādaśapaṇo daṇḍaḥ, anyatra svayaṃvādibhyaḥ // iti dāsakalpaḥ /

Selling or mortgaging the life of a male or a female slave once liberated shall be punished with a fine of 12 panas with the exception of those who enslave themselves. Thus the rules regarding slaves.