A man who swears he will beat his slave: what suffices him?

Abdullah bin Ubayd bin Umayr narrates about his father that he used to release himself from his oath (to beat his slave) by a mild beating, a blow lighter than a real beating.

Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: If someone swears that he will certainly beat his slave, its expiation is not to do it, and there is a good deed for him in this expiation.

Sayyiduna Ibn Abbas (may Allah be pleased with them both) was asked about a man who vowed that he would strike his slave thirty lashes or more. He said: "He should gather all the lashes together and strike him with them a single time."