Or HaChaim on Exodus 3:21/אור החיים על שמות ג׳:כ״א

1 לא תלכו ריקם, "You will not leave empty-handed." At this point the Torah already foreshadowed the legislation applicable when a Jewish servant is to be released by his master. The Torah commands in Deut. 15,13-15 that you must not set him free empty-handed but that you have to provide him with a stake so that he can establish an economic base for himself. Presumably, this is also measure for measure. What G'd did for you, you in turn are to do for the servant you set free.