Adam Clarke on Leviticus 19:20

A woman that is a bondmaid - Had she been free, the law required that she should be put to death; (see Deu 22:24); but as she was a slave, she is supposed to have less self-command, and therefore less guilt: but as it is taken for granted she did not make resistance, or did consent, she is to be scourged, and the man is to bring a ram for a trespass-offering.