Evolution of an idiom:
Formulation "kindness is free of…" appears in a few sources in the early 19th century, as critiques of solicited mercy, affectation, alloy (impurities).
Then, in 1849, an apologetic on mercy of "the Lord": "he is a debtor to none of mankind, and that whatever he gives is a gratuitous benefit, and then that his kindness is free, so that he can confer in on who he pleases".
https://books.google.com/books?id=R6kGAAAAQAAJ&pg=PA356&dq=%22kindness+is+free%22&hl=en&newbks=1&newbks_redir=0&sa=X&ved=2ahUKEwjRvdTCyvyPAxVGD1kFHSMHEjgQ6AF6BAgHEAM#v=onepage&q=%22kindness%20is%20free%22&f=false
Here, kindness is free of obligation (debt), given freely (gratuitously). Not that it costs nothing, but that it is not transactional. Note this is kindness of God, not of man.
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