TLDR: In principle, you convert the amount of foreign inheritance tax creditable in France according to the exchange rate applied on the Paris market on the day that tax was actually paid.
The conversion rule
You must assess the amount of foreign tax creditable in France in the currency in which you paid it. You then convert this amount into euros by applying, in principle, the exchange rate applied on the Paris market on the day the foreign tax was actually paid.
The rule applies to amounts of foreign tax that can effectively be credited against the inheritance tax payable in France. The source does not specify which particular quotation should be used when payment was made in a currency other than the euro.
When foreign tax can be credited
Article 784 A of the French General Tax Code provides that, in the situations referred to in 1° and 3° of Article 750 ter, transfer duties paid free of charge outside France may be credited against the tax payable in France.
This credit is limited to tax paid on movable and immovable property located outside France. The conversion into euros therefore concerns only the amounts falling within this credit limit.
What the concept of tax paid outside France covers
For this rule, taxes paid “outside France” are those paid in a country or territory other than metropolitan France and the overseas departments. This concept also includes French overseas territories.
Limits of the rule
The exchange rate must be assessed on the date the foreign tax was actually paid, and not on another date indicated in the source. The information provided specifies neither a particular quotation within the Paris market, nor a filing procedure, nor a specific form or deadline for carrying out this conversion.