How to Set Currency When Creating an Invoice

Hey everyone, 

I'm running into an issue here where I'm creating an invoice in Sage and it's being created properly, but it doesn't seem to want to respect either a Currency object or the Currency Id. This account is for a company that is based in Canada, so I guess the default behavior is to use CAD, but when they sell in the US it needs to be in US dollars.

Here's the request that I'm sending. When it responds back with the invoice, it's still in CAD. I'm hoping someone can point out what I'm doing wrong. Thanks in advance! 

Using the Currency Id: 

{
  "sales_invoice": {
    "contact_id": "2ae9dd2e69e242cf8e195fd0ad662d9c",
    "total_discount_amount": 0,
    "date": "2020-01-08",
    "invoice_lines": [
      {
        ....
      },
      {
        ....
      }
    ],
    "currency_id": "USD"
  }
}

Using the currency object: 

{
  "sales_invoice": {
    "contact_id": "2ae9dd2e69e242cf8e195fd0ad662d9c",
    "total_discount_amount": 0,
    "date": "2020-01-08",
    "invoice_lines": [
      {
        ...
      },
      {
        ...
      }
    ],
    "currency": {
      "id": "USD",
      "displayed_as": "US Dollar (USD)",
      "$path": "/currencies/USD"
    }
  }
}