Problems with user Custom fields


I have a big problem with user custom fields. I will describe it that way:

I habe a use with the real name “Max Mustermann” with the id #13159. He
possesses a user custom field with the id #17 and the name “test”.
It is a text field and its value is “foo”. It is supposed to be empty
for all other users.

One day, I change that costumfield of that user to “bar” and I have a
collection of Users called $Users which I intend to limit that way:

“OPERATOR” => “=”,
“VALUE” => “foo”);

The very trouble is that this collection still enlists "Max Mustermann"
as “foo” is a former customfield value of the customfield #17.

This is not very helpful. Nevertheless, I want to enlist all users
whose CURRENT value of customfield #17 is “foo”. Is it possible? Or
is it impossible?

Thanks in advance,