I have turned down several jobs that contain a penality clause, it relies too much on others completeing thier task and I'm not happy when it's out of my control. I would suggest theirs terms are unreasonable, as you are directly contracted to them and not a third party. I'm aware a lot business is built on trust and this seems to me to break that trust, it maybe you continue for years with no problem but it suggests to me they may fore see a reason to need this clause. If it was me I would check if this is enforcable and if not let them know or walk away.