Initially, after the first Muslim conquests in the 7th century, kharaj usually denoted a lump-sum duty levied upon the conquered provinces and collected by the officials of the former Byzantine and Sasanian empires, or, more broadly, any kind of tax levied by Muslim conquerors on their non-Muslim subjects, dhimmis.
When the Muslim traders went to foreign lands for the purposes of trade they had to pay a 10% tax to the foreign states.