Is it permissible to dispose of silver currency before paying Zakat on it, then pay the Zakat after disposing of it?

There is no blame upon you for what you have done. If you gave the riyals to your brother before a year passed and zakat became due on them, then you are not obligated to pay zakat on them, and it will be considered charity for you, and zakat will be due on your brother. If you gifted them to him after aalians passed and zakat became due on them, then your obligation has been discharged by giving them out.