Security Question

I've got a question about security that may reflect some misconception on my
part -- but here goes anyway:

I think that the XML payload of the response from a web service -- or indeed
I suppose the message that invokes it -- may be validated by a schema.  If
so, that schema can add data via defaults and/or fixed values.  How is this
secured?

Let me be more specific with a contrived example:  Suppose we are purchasing
widgets via a web service and in the XML document you specify "1" for the
amount to purchase.  However, suppose the schema has a default value of
"Each" that explains the meaning of the "1".  Now suppose that either from
malicious tampering or through the use of a schema intended for a different
audience that default (which is on the seller side) is changed to "dozen".
Now the "1" really means 12 items, which is a lot more expensive.

This is obviously contrived and dumb, but I think it illustrates the fact
that schemas can affect data.

So how is this secured?  Can the buyer in the context of the message
unambiguously specify what schema must be used for validation and have some
sort of check that it was the right one?  Can it be secured?