There is a subtle difference between a contrary provision or exception and an opt-out.

BTW, in my (limited) experience, the translation of opt-out works 9 times out of 10 for dérogation.

Notwithstanding the unfortunately similarly sounding "derogatory", there is an exact equivalent in EN: a derogation i.e. "an exception to the general rule".

"unless derogations have been agreed in the purpose order ...."

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Note added at 13 hrs (2019-05-19 02:42:44 GMT)
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there is here one sample where "derogations" are exceptions to some usual / general contractual conditions, NOT exceptions to some legislation:

The documents used by the Council of Europe depend on the strategic importance of the tenders to be awarded (eg: is the tender divided into several packages? are specific clauses – deadlines or penalties - necessary?) and/or their financial scale (eg: value of the tender).

They fall into one of two groups:

-Tenders < 25 000 EUR: order letters and any financial appendices, General Purchasing Conditions and/or service provision simplified
-Tenders > 25 000 EUR:
--Act of Engagement (AE): document in which the supplier commits their company for a fixed amount;
--General Conditions (CG): the General Purchasing Conditions of the Council of Europe, or for certain purchases, the General Conditions for Works; Supplies and Services; Intellectual Services; or IT Services, which override any other Sales Conditions of the Organisation's suppliers.
--Special Conditions (CS): document listing any derogations from the General Conditions;
--Technical specifications (ST): document conveying the Council of Europe's technical needs;
--Breakdown of the overall, all-inclusive price (DPGF): used to list the supplier's bids for all-inclusive needs;
--Schedule of unit prices (BPU): used to list the supplier's bids for needs broken down by unit;
--Tender rules (RC): non-contractual document providing suppliers with all the rules to be followed when replying to a call for tenders.

https://www.coe.int/en/web/portal/contracts

Given that this sample comes from an international organisation that certainly can afford a good legal department, it should be given some weight ...


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Note added at 13 hrs (2019-05-19 02:53:04 GMT)
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in this ST:

Les Spécifications du Logiciel élaborées par le Prestataire seront soumises à [...] qui disposera, à défaut de stipulations dérogatoires fixées dans le Bon de commande, d'un délai de cinq jours ouvrés pour les approuver en signant contradictoirement un procès-verbal de recette ou pour les refuser en faisant connaître par écrit ses observations.

the "derogation / exception to our general rule" here is that the client was allowed at the time of accepting the purchase order more than the usual/normal 5 working days to object to the quality of delivered software.

.... "unless derogations have been agreed in the purchase order ...."