In, say, Germany a verbal contract is as good as a written contract, as far as I know. (And both situations aren't really applicable to the Netherlands, anyway.)
In the German system the oral addition stands above the written contract. The written contract is considered boiler-plate by the courts and the oral addition is more specific, and bears more weight.
Of course all parties have to agree to amend a contract. So just the employer changing their mind wouldn't cut it.
The Netherlands is a civil law country, so the German standard might be closer to what they have than the common law American standard.
Labour relations and law are of course a strange thing in themselves.