Ex happily agreed, and when Sat came, called me and said "Sorry, changed my mind."
When it was brought up in court, the judge said "This is why you should never make verbal agreements outside the court order."
Now, that's not to say that the judge wouldn't take it seriously IF there were several instances of you giving up time to mom to accomodate plans with the child, and her not reciprocating - but not as contempt - only as evidence of a pattern of behavior.