Does ijtihad contradict Allah's saying: "This day I have perfected for you your religion and completed My favor upon you and have approved for you Islam as religion"?

There is no contradiction between the scholarly ijtihad (independent reasoning) and Allah's saying, "This day I have perfected for you your religion" (Quran 5:3). The verse explains Allah's grace upon Muslims by perfecting their religion, and scholarly ijtihad is part of the religion itself, provided it is based on the Quran and Sunnah and their objectives.