Is it permissible to exchange ten paper dirhams for nine metal dirhams, and is that considered usury (riba)?

Paper currencies are treated like gold and silver; they are a price and value for things, and the same rules that apply to gold and silver apply to them. The currency of each country is considered a single type, so it is not permissible to trade them unequally, such as selling ten units for nine, as this falls under riba al-fadl (usury of excess).