Solution

Upgrade Avatax API to v23.2.

Added User Authorizations:

Support for native SAP External Tax Calculation with foreign currency.
In this example the document is set to use Australian Dollar (AUD).
Lines are mixed using the unit price in USD and AUD.
Tax Amount is set with the document currency (AUD) and document Total Tax is set in AUD.
Working on SAP 10 (10.00.191) FP 2202. If the tax is not set to the document using a previous SAP version, you may need to upgrade your SAP installation to use this feature.