the passage noted has NOTHING whatsoever to do with tithing, nothing. its a long discussion about the covenant of Abraham and it about the fact that "seed" here should not be taken as physical seed by spiritual. its a long discussion. no where in the entire book does paul mention tithing nor does he use this "creative interpretation" as you put it to support tithing.
in fact i don't think paul ever addressed the notion of tithing in any of his letters, but i could be wrong about that.