Why does it say that gold will be in place of copper?
Rashi: It will replace the copper that they took from you. Radak - I will put in their hearts to pay to you many times what they and their fathers took from you.
Malbim: Do not think that they will pay to you taxes amidst coercion. When you request a tax of copper [or iron], they will give gold [or silver]. Do not think that you will lack copper or iron. When you request wood [or stones], they will give copper [or iron]!
What is the meaning of "v'Samti Fekudasech Shalom [v'Nogeshayich Tzedakah"?
Targum Yonasan: I will make your leader Shalom, and your rulers meritorious. Rashi - Fekudasech are your Pekidim.
Rashi citing a Midrash: The Pekudos (commands) that came on you, and the Nogeshim (taskmasters) who pressed you, they will be for Shalom and Tzedakah.
Radak: In place of the appointees that the Nochrim put on you to collect taxes, I will give to you Shalom and Tzedakah. You will not have any more Pekidim and Nogeshim.
Malbim: Your appointment over Nochrim, they will not consider that they are subjugated to pay tax and do not have Shalom. Rather, this is their Shalom! Your appointment over them is via their desire, and not via war. What presses them to pay taxes to you is like one who does a Mitzvah of giving Tzedakah.
