Scholars of Islamic Finance quote the Hadith:
المسلمون على شروطهم إلا شرطا حرم حلالا أو أحل حراما
Where is this Hadith located and what is the strength?
Imam Tirmidhi (rahimahullah) has recorded this Hadith and has graded it authentic (hasanun sahih).
(Sunan Tirmidhi, Hadith: 1352)
Imam Bukhari (rahimahullah) has cited the first part of the Hadith ta’liqan (without a chain).
(Sahih Bukhari, before Hadith: 2274 and Fathul Bari, Hadith: 2274)
Muslims abide/are held by their conditions, except those conditions that make the lawful unlawful, or the unlawful lawful.
Note: I have merely answered your question regarding the authenticity of the Hadith. This should therefore not be viewed as a Fiqh ruling. Kindly contact a reputable Mufti/Darul Ifta for clarification regarding the application of this Hadith.
And Allah Ta’ala Knows best.
Answered by: Moulana Suhail Motala
Approved by: Moulana Muhammad Abasoomar
Checked by: Moulana Haroon Abasoomar