IMO, two way are possible :
modifying so that negative number are accepted in <iGlobalReligionCommerce>
AND allowing <iGlobalReligionCommerce> to be used for any non-state religion X (if <iGlobalReligionCommerce> is used somewhere else I hadn't found, it might not be a good idea)
or creating a <iCityReligionCommerce> ?? function allowing the same trick
as vanilla civ has no <iCityReligionCommerce> values, it shall not change the vanilla civ game, but may improve modability
As I do not know how to modify SDK, I leave you with the idea, and if you find smarter ways, it will even be better
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so the real question is : "could you allow negative iCommerce values for religions?"