Interestingly enough, I'm struggling with a similar phrase in a document I'm working on. The meaning is clear enough; no stipulations can be made that supersede the law unless they are in the buyer's favor. How best to express that in elegant English, I'm not certain.

..unless it be to the advantage of the purchaser.

This seems to be almost directly taken from Section 7A of the Sales Law. (Apartments), 5733-1973 (חוק המכר (דירות), תשל"ג-1973)
(א) אין להתנות על הוראות חוק זה, אלא לטובת הקונה או קונה המשנה.
http://www.moital.gov.il/cmsTamat/InternalPage.aspx?FRAMELES...

The only EN translation of that sentence I can find online doesn't read too well to me:
http://www.israelinsurancelaw.com/contract-laws/sale-apartme...

I had a similar sentence recently which read something like:
"which cannot be stipulated against unless to the advantage of the Insured", the HE was: "שאין להתנות עליו אלא לטובת המבוטח".

This is common in legal contexts—for example, with corporate codes that seek to circumvent legal requirements by qualifying them in various ways, e.g. making them conditional on certain circumstances.