Thank you for reaching out to the Community! I’m happy to provide some assistance with Facebook Shop, to get this resolved. While customers will be taken from Facebook to your Shopify Store to complete the checkout process, you may be able to update the currency populating on Facebook directly. I recommend Contacting Facebook Support for confirmation on this, but can check the Data Feed Catalog to see if currency adjustments are available.
For more information on setting up Facebook Shop, I encourage you to check our Facebook Shop Documentation for further details on this.
Please let me know if you have any other concerns.