Though the verse did not explicit say whether the slave girl was an Israelite or a gentile, but as she could be redeemed, she should be an Israelite. According to Exodus 21:7-11 regarding the law of woman slave, it said;
7 “If a man sells his daughter as a servant, she is not to go free as male servants do.
8 If she does not please the master who has selected her for himself, he must let her be redeemed. He has no right to sell her to foreigners, because he has broken faith with her.
9 If he selects her for his son, he must grant her the rights of a daughter.
10 If he marries another woman, he must not deprive the first one of her food, clothing and marital rights.
11 If he does not provide her with these three things, she is to go free, without any payment of money. (Exodus 21:7-11 NIV)
Now let's see the man and the slave girl described in Leviticus 19:20-22
20 “‘If a man sleeps with a female slave who is promised to another man but who has not been ransomed or given her freedom, there must be due punishment. Yet they are not to be put to death, because she had not been freed.
21 The man, however, must bring a ram to the entrance to the tent of meeting for a guilt offering to the Lord.
22 With the ram of the guilt offering the priest is to make atonement for him before the Lord for the sin he has committed, and his sin will be forgiven.
The slave girl was not to be put to death because she was not a free woman. This had been answered clearly. But what about the man, why was he not put to death? Who was the man?
As the event was taken place in the master house, the man was either the master, or the master's son, or the male slave/servant. It was unlikely to put the master or the master's son to dead, for in the situation, the slave girl was still deemed as a property of the master. So the law was likely to protect the male slave/servant. Why was it a protection to him? For him was still a property of the master, that would be killed by his master for a reason.
The male slave, unlike the female, was allowed to leave his master in the seventh year (Exodus 21:2), back to their clans and inherited the land of their own (Levitus 25:41). If he was killed illegally, his bloodline would be cut off that was forbidden by the Lord.
Then, did he commit a crime that could be killed? Review the law about this in Deuteronomy 22:13-30, in below was the law regarding unmarried women;
23 If a man happens to meet in a town a virgin pledged to be married and he sleeps with her,
24 you shall take both of them to the gate of that town and stone them to death—the young woman because she was in a town and did not scream for help, and the man because he violated another man’s wife. You must purge the evil from among you.
25 But if out in the country a man happens to meet a young woman pledged to be married and rapes her, only the man who has done this shall die.
26 Do nothing to the woman; she has committed no sin deserving death. This case is like that of someone who attacks and murders a neighbor,
27 for the man found the young woman out in the country, and though the betrothed woman screamed, there was no one to rescue her.
28 If a man happens to meet a virgin who is not pledged to be married and rapes her and they are discovered,
29 he shall pay her father fifty shekels of silver. He must marry the young woman, for he has violated her. He can never divorce her as long as he lives.
The man should not be put to death because
- The slave girl was likely not a virgin anymore as Exodus 21:8 implied
- It was not a rape crime as Levitus 19:20 did not say so
Deuteronomy 22:28 may be the closest law, but she was a slave girl and not a virgin of free woman. Even was that, he was not put to dead according to this law.