Nobody but the parties to that contract are, so far as I understand, considered qualified to judge what constitutes fair value.
Unless, of course, you enter into a series of artificial transactions which the tax man decides is designed solely to avoid tax, in which case the tax man considers himself qualified to judge that the whole caboodle has no legal validity whatsoever (for tax purposes). Quite possible the CAA has fewer powers than the tax man in this regard, but talking to a lawyer might indeed be a good idea.