All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
The basic principle is that it is an obligation to abide by the stipulations that do not violate the sharee'ah in all contracts. The Prophet said, "Muslims are bound by their stipulations, except a stipulation that makes what is lawful unlawful, or makes what is unlawful lawful." [At-Tirmithi graded it hasan saheeh (good or sound)] Ibn Taymiyyah wrote, "Stipulations that do not violate the Islamic sharee'ah are valid and binding in all contracts..." [Al-Ikhtiyaaraat]
Therefore, the lawful stipulations in any contract agreed upon by the contracting parties are binding on them.
The stipulation to which you referred in the question does not violate the sharee'ah; therefore, it is a lawful stipulation and you are obliged to abide by it and cannot violate it.
Allaah knows best.