אַ֥ךְ אִם־י֛וֹם א֥וֹ יוֹמַ֖יִם יַעֲמֹ֑ד לֹ֣א יֻקַּ֔ם כִּ֥י כַסְפּ֖וֹ הֽוּא׃ (ס)
但僕人若能存活一天或兩天，主人就不被治罪，因為僕人是他用銀錢買來的。
Rashi
אך אם יום או יומים יעמד לא יקם NOTWITHSTANDING IF HE CONTINUE A DAY OR TWO, HE SHALL NOT BE AVENGED — If the master is free of the death penally in case of the servant remaining alive one day, is it not logical that he is free if he lives two days (what need is there for Scripture to add יומים)? But it adds it to show that the term יום is to be defined by יומים — “a day that is like two days”. And what kind of day is thereby meant? A period of twenty-four hours (a day which comprises a part of two separate days) (cf. Mekhilta).
Rashbam
כי כספו הוא, and it is in order to administer a bodily blow in order to discipline him.
Sforno
כי כספו הוא, and it is up to him to discipline him. Sometimes a slave’s insubordination continues until the master is forced to administer a strong blow. We have read of such in Proverbs 17,11 ומלאך אכזרי ישלח בואך מרי יבקש רע, “an evil man seeks to rebel; therefore a cruel messenger will be dispatched against him.”