You should firstly have bank accounts in the currencies which is not in your functional currency.
In my organisation for example, we have dollar accounts, pound Sterling accounts, euro accounts and local/home currency accounts.
Proper accounting requires that you have Bank accounts in these currencies. In Manager, you can even create a cash account in any currency available in the currencies list.
In receiving money, receive money into an account (cash/bank) with a currency that is same as the currency the customer is paying you with.
You will see a field with “Optional” as default text (field is located after tracking codes field), enter local currency equivalent as at that date there. Gains and losses on the exchange rate will be correctly calculated by manager and posted correctly.
So what you need is to have bank/cash accounts in the other currencies set up. Applies to when paying money too.