وإذا قيل لهم أنفقوا مما رزقكم الله
And when it is said to them: "Spend of that with which Allah has provided you,''
means, when they are told to spend of that which Allah has provided to them on the poor and needy among the Muslims,
قال الذين كفروا للذين آمنوا
those who disbelieve say to those who believe,
means, about the believers who are poor,
i.e., they say to those believers who tell them to spend on the needy,
أنطعم من لو يشاء الله أطعمه
Shall we feed those whom, if Allah willed, He (Himself) would have fed,
meaning, `those whom you are telling us to spend on, if Allah had wanted to, He would have made them independent and fed them from His provision, so we are in accordance with the will of Allah with regard to them.
إن أنتم إﻻ في ضﻼل مبين
You are only in a plain error.
means, `by telling us to do that.'