Why should I enter the customer currency while creating a customer or a vendor?
If you deal with customers or vendors who are located outside your country, you may receive or make payments in a foreign currency. You can associate that currency with the customer or vendor to create transactions with that currency automatically.
To associate a currency to a customer:
Go to the Sales module and click Customers or go to the Purchases module and click Vendors.
Click the + New button in the top right corner of the page. Enter all the necessary details and in the Other Details section, select the Currency from the currency dropdown and click Save.