The rule concerns a word that was already used as a proper name in the non-Arabic language. For example, دِرْهَمٌ is a borrowed common noun and remains declinable; foreign origin alone does not complete the proper-name + foreignness pair.
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Foreign Origin Alone Is Not Enough
Causes of Ghayr Munsarif: Proper Names and Foreign Origin
The rule concerns a word that was already used as a proper name in the non-Arabic language. For example, دِرْهَمٌ is a borrowed common noun and remains declinable; foreign origin alone does not complete the proper-name + foreignness pair.