Yes, if the seller agrees to clean up debris but does not, then that is a breach of contract. You can refuse to close on the deal until the task is performed. If they simply refuse to perform.. then you have two options. 1. close on the deal anyway and clean it up yourself. ( you may be able to sue latter for your efforts... but really? would it be worth it?? consult an Attorney on that.) 2. You can void the contract for non performance and walk away.