Sterling work

I have a self-employed client who received a considerable amount of money from an overseas company for consultancy work. They paid him in a foreign currency directly into his foreign bank account.

The client’s invoice was issued some time after the work was completed (with predictable and now solved VAT problems). In that time sterling fell considerably against the foreign currency.

He has kept the money for some time in the overseas bank account and the pound has fallen further against the foreign currency.

I assume that any profit on the dollars will be subject to capital gains tax as this is not an adventure in the nature of trade.

However I am unsure as to what the base cost and acquisition cost of the foreign currency should be. Should it be the exchange rate on the (admittedly delayed) VAT invoice?

Alternatively should it be the exchange rate...