What if your in-laws wrote the mahr amount without consulting you as a bride?

If you were not consulted about the mahr amount, then it reverts to mahr al mithl.

You should have been consulted on that and you had the responsibility to find that out, but that also does not disenfranchise you from taking that which is yours and is reasonable.

And so if a small amount which was unreasonable and outside of the confines of what would be mahr al mithl, the like thereof of the mahr that you or women in your family would have received.

Then yes, you can demand that, but you should not demand more than that.