manusmriti 10 (legal rules for the four castes)

Brahmana will teach not the other castes:  1. Let the three twice-born castes (varna), discharging their (prescribed) duties, study (the Veda); but among them the Brahmana (alone) shall teach it, not the other two; that is an established rule.

61. But that kingdom in which such bastards, sullying (the purity of) the castes, are born, perishes quickly together with its inhabitants.

80. occupation: Among the several occupations the most commendable are, teaching the Veda for a Brahmana, protecting (the people) for a Kshatriya, and trade for a Vaisya.

96. the king shall deprive:  A man of low caste who through covetousness lives by the occupations of a higher one, the king shall deprive of his property and banish.

117. interest prohibited but : Neither a Brahmana, nor a Kshatriya must lend (money at) interest; but at his pleasure (either of them) may, in times of distress when he requires money) for sacred purposes, lend to a very sinful man at a small interest.

 122. this life and the next: But let a (Sudra) serve Brahmanas, either for the sake of heaven, or with a view to both (this life and the next); for he who is called the servant of a Brahmana thereby gains all his ends.