Yes the bookies cheat, this is not restricted to IG, they all do it.
Sometimes the spread is not enough for them to get an edge over the punters so they need that little extra.
This is most true on markets where they cannot easily hedge your bet due to various reasons.
How did your account go so negative if you were betting so small?
Best to just pay up and move on.
The "debt" is less than 5K.