Is now available that law which God told us that every man give 10% of his income? Why in the old testament that lend a non-Jew a Jew could ask ask why usury on money? but if a Jew borrow another Jew was not allowed to ask usury … What is the explanation?
1) There is no biblical obligation to give 10% of a person’s income to charity.
The biblical obligation refers to a person produce (in the Land of Israel), from which 10% must be given to the Levites and another portion to the Priests.
Based on this biblical injunction, there is a rabbinic custom (or possibly a rabbinic obligation) to give 10% of one’s income to charity. Although inspired by the scriptural model, this is not a full biblical command.
2) Scripture relates differently to Jews and to non-Jews. Although there are rights and obligation that apply to non-Jews too, the set of obligations applying to a person’s fellow Jews is broader, because the Jewish people is seen as a type of family unit for which each member has a heightened sense of responsibility.
Therefore, among Jews there is an obligation for a Jew to offer his fellow an interest-free loan, and there is therefore a prohibition against taking interest. This prohibition does not apply to non-Jews, for whom there is no obligation to give an interest-free loan.