The foreign_exchange_rate object is how Modern Treasury represents exchange rate information between two currencies with high precision. The rate itself is not a resource that can be queried for, but will serialized as part of the object it applies to.

Attribute	Type	Description
base_amount	`integer`	Amount in the lowest denomination of the `base_currency` to convert, often called the "sell" amount.
base_currency	`string` ISO 4217	Currency to convert, often called the "sell" currency.
exponent	`integer`	The exponent component of the rate. The decimal is calculated as `value` / (10 ^ `exponent`)
rate_string	`string`	A string representation of the rate, e.g. `"1.234"`
target_amount	`integer`	Amount in the lowest denomination of the `target_currency`, often called the "buy" amount.
target_currency	`string` ISO 4217	Currency to convert the `base_currency` to, often called the "buy" currency.
value	`integer`	The whole number component of the rate. The decimal is calculated as `value` / (10 ^ `exponent`)