Talk to your agent and attorney and see if they can help with a compromise but if not, don't risk losing you deal.
Unwavering Commitment to Service
If by you needing another week means that you will be past the closing date of the contract, there's not much you can do unless the buyer agrees to extend the contract.
I would ascertain the reason why and see if there's a way you can negotiate for the extra week.
I'm not a lawyer; but if things do not work out, you may want to seek an opionion from a Real Estate attorney regarding contractual obligations.