fight it because if they told you they had a warrant and that was the sole cause the entered the house (the basis of you MIP charge I'm sure), then the fact that they announced they had a warrant (and didn't) adds up to false pretenses. Meaning anything they found, confiscated, and charged people with is basless because they lied in order to get the charges. Good luck getting it to hold up in court though, unless you have witnesses as well, the cops will conveniently forget the specifics of the night and deny any wrong doing (even though the ymost likely don't really remember - and I have testified on behalf of a retired MA state Trooper, so I know the tricks and that they are trained to testify, and how they do it).
Word to the wise, never let a cop in until you either see a gun or a warrant, if they say they have one, they better have it on hand.
life is too short to have any regrets