What is the evidence for gold and silver being considered money now, and is it permissible to sell gold for US dollars on credit to a Muslim (by a Muslim) for a non-believer in non-Muslim lands, knowing that gold and silver have lost their monetary function at present?

Usury (riba) in gold and silver is forbidden by text (Qur'an and Sunnah), and it is forbidden in both of them in its two categories: riba al-fadl (usury of surplus) and riba an-nasi'ah (usury of delay). Riba al-fadl is forbidden when the جنس (category/type) is the same, and riba an-nasi'ah is forbidden even if the categories differ.

The prohibition of usury in the two currencies (gold and silver) is established by text, while the reason for this prohibition being "their function as price/medium of exchange" (ath-thamanīyah) is an inferred matter. Usury is forbidden in banknotes by analogy to the two currencies due to a common underlying reason, which is ath-thamanīyah.

Riba an-nasi'ah is forbidden in all (gold, silver, and banknotes), and riba al-fadl is forbidden when the جنس (category/type) is the same (gold is one جنس, silver is another جنس, and the currency of each country is a جنس). It is not permissible to have a surplus (at-tafāḍul) within the same جنس, even if its denominations differ. Nor is it permissible to have a delay (an-nasā') in monetary transactions except by immediate exchange (at-taqābuḍ).

The misconception that ath-thamanīyah has been withdrawn from the two currencies, making them mere commodities, is incorrect. This is because the reason of ath-thamanīyah in the prohibition of usury is an inferred matter, whereas the ruling itself is established by text. As for metallic currencies whose value has completely collapsed, they revert to being like other metal pieces. They can be sold for each other, even with a surplus, and bought with immediate or deferred payment, as they are no longer functioning as a medium of exchange for everything priced.