If someone swears to beat his slave, how much beating is sufficient?

Hazrat Abdullah bin Ubaid bin Umair narrates from his father that a person will be absolved of his oath for lightly (slightly) beating his slave.

Hazrat Ibn Abbas (may Allah be pleased with both of them) says that if a person swears that he will definitely beat his slave, then its expiation is not to do it, and for him, doing good is the expiation.

Hazrat Ibn Abbas (may Allah be pleased with both of them) was asked that a person has made a vow that he will strike his slave with thirty or more lashes. He replied: He should gather all the lashes together and strike him with them at once.