It is narrated from Ibn Sirin (may Allah have mercy on him) regarding Shurayh (may Allah have mercy on him) that if someone secretly agreed upon a dowry (mahr) for marriage and publicly declared a higher amount, then Shurayh (may Allah have mercy on him) considered the secret (mahr) valid and the public one invalid. Hisham said: "This is also our opinion."
Hadith Referenceسنن سعید بن منصور / كتاب النكاح / 2175
Hadith Takhrij«أخرجه سعيد بن منصور فى «سننه» ترقيم الدرالسلفية برقم:، 998، 1000، وابن أبى شيبة فى «مصنفه» برقم: 16361»