You are right its not literally in the Greek text, but the meaning is as NIV presented (thats why it is not always best to read literal translations):
[not here] εἰ δὲ ἡ ἀλήθεια τοῦ Θεοῦ ἐν τῷ ἐμῷ ψεύσματι ἐπερίσσευσεν εἰς τὴν δόξαν αὐτοῦ, τί ἔτι κἀγὼ ὡς ἁμαρτωλὸς κρίνομαι;
καὶ μὴ καθὼς βλασφημούμεθα καὶ καθώς φασίν τινες ἡμᾶς λέγειν ὅτι Ποιήσωμεν τὰ κακὰ ἵνα ἔλθῃ τὰ ἀγαθά; ὧν τὸ κρίμα ἔνδικόν ἐστιν.
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Paul frequently stylizes himself into various positions to attack it later. Its his style of writing. Sometimes, he places himself "under the law" to prove something, even though it is not his claim that he actually is etc.
[not here] εἰ δὲ ἡ ἀλήθεια τοῦ Θεοῦ ἐν τῷ ἐμῷ ψεύσματι ἐπερίσσευσεν εἰς τὴν δόξαν αὐτοῦ, τί ἔτι κἀγὼ ὡς ἁμαρτωλὸς κρίνομαι;
καὶ μὴ καθὼς βλασφημούμεθα καὶ καθώς φασίν τινες ἡμᾶς λέγειν ὅτι Ποιήσωμεν τὰ κακὰ ἵνα ἔλθῃ τὰ ἀγαθά; ὧν τὸ κρίμα ἔνδικόν ἐστιν.
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Paul frequently stylizes himself into various positions to attack it later. Its his style of writing. Sometimes, he places himself "under the law" to prove something, even though it is not his claim that he actually is etc.