You need to be comfortable that every contract you sign is clear and the other parties know exactly what the terms require. A modification clause restricts parties from claiming that anyone agreed to change a term in the contract, unless of course all parties have agreed to that change in writing. The language might say something like, the parties agree that no modification of any term of this agreement is enforceable. Unless it is written out and signed by all parties, consult a business lawyer to know the best language that will suit your needs.