Sheep-herder=is a person whose job is to take care of sheep to guide and make sure a group of people somewhere. Hence guardian / custodian shop. eg:-Sheep-herders should be guardians / custodians of the (---------).
Here is a question that I'd appreciate you for clarifying. Is your company(the Citibank) the custodian bank of this foreign exchange broker? Do you receive the brokerage business of the foreign exchange broker?