John Calvin Commentary Colossians 4:1

"Masters, render unto your servants that which is just and equal; knowing that ye also have a Master in heaven." — Colossians 4:1 (ASV)

Masters, what is just. He first mentions what is just, a term by which he expresses that kindness concerning which he has given instruction in the Epistle to the Ephesians (Ephesians 6:8). But as masters, looking down, as it were, from on high, despise the condition of servants, so that they think they are bound by no law, Paul brings them under control, because both are equally subject to the authority of God. This is why he mentions that equity.

And mutual equity. Some understand it otherwise, but I have no doubt that Paul here used ἰσότητα to mean analogical or distributive right, as in Ephesians, τὰ αὐτὰ, (the same things). For masters do not have their servants so bound to them that they do not, in turn, owe them something, since analogical right is to be in force among all ranks.