How do we respond to the specious argument that usury is forbidden to people but permissible for God, based on the verse, "Who is it that would loan Allah a goodly loan so He may multiply it for him many times over?"

A حسن (goodly) loan to Allah refers to the good deeds one performs seeking Allah's reward, such as charity (صدقة) and spending [in His cause] (نفقة). Allah multiplies their reward out of His grace and generosity. He is self-sufficient; the benefit of those who do good does not increase Him, nor does the shortcoming of wrongdoers harm Him. As stated in the Hadith Qudsi, creatures cannot reach His harm or His benefit, and their obedience does not add anything to His dominion, nor does their disobedience diminish anything from it. This is like a father who forbids usury to his children but promises to multiply the good deed of anyone among them who excels. This does not mean that he has permitted usury for himself.