It depends on whether the piece of paper--what is says, how it was signed, when and under what circumstances it was signed, etc.--meets the criteria to be a pre-nuptial agreement. You should bring it to a family or matrimonial law attorney to review, so as to understand its enforceability. With any sort of contract or agreement, the details matter as to enforceability, so we can't say whether this one will hold up--only that it might, and that it's worth your time to consult with a lawyer about it.