I think your sentence has other problems of number, and the meaning is obscured. ) them is a matter of diplomacy rather than friendship. ) them is rather a matter of diplomacy than friendship.
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IMGWhy did you use "a" before "relationship"? It's uncountable noun, isn't it?It is normally countable. That is part of what makes your original sentences unclear.
IMGWhat if I say "Relationships with them are rather diplomacy than a real friendship."? Does it sound OK?Yes, but again you're back to plural 'relationships', singular 'friendship' and uncountable 'diplomacy'!