Capital gain with deferred payments not in sterling...

I have a UK resident client who is excersising share options in a Canadian company.  The proceeds are to be paid over the next few years, assuming certain contingencies are met (remains in employment etc...).

The deferred payments are to sit in a CAD$ account, and paid over in 1 + 2 years.

(The client lives in the UK and uses sterling accounts)

I appreciate the full gain is to be recognised now, and will have to base it on the exchange rate at point of disposal, but is there any scope to amend the tax comp later if the exchange rate changes significantly over the next couple of years?

Any input appreciated - thanks