Start with the original amount
Keep the currency code beside the amount on the source document. A symbol alone can be ambiguous. Retain the invoice as issued, including its original currency, instead of replacing that amount with a converted figure in your working notes.
Explain the conversion
When a reporting amount is needed, record the rate, its date and the source you used. Check which direction the rate expresses. A rate of 1.10 USD for 1 EUR is not the same as 1.10 EUR for 1 USD. The arithmetic should be reproducible by the next reviewer.
Keep settlement separate
The amount received can differ because of timing, conversion or fees. Match the settlement to the right invoice, then explain the difference using the provider record. Do not silently change the invoice amount to force the records to agree.
Ask the right local question
A market being supported does not settle the tax or reporting treatment of a particular transaction. Bring the source record and rate details to the person responsible for your accounting. Keep jurisdiction-specific decisions distinct from the mechanics of converting an illustrative amount.