Can the imam begin the prayer before the iqamah is finished?

In addition to the obligatory, necessary, and Sunnah acts of prayer, there are also etiquettes. According to Imam al-A'zam, it is among the etiquettes of prayer that the imam begins the prayer at the moment when "Qad qāmatiṣ-ṣalāh" (the prayer has begun) is announced for congregational prayer. By doing so, the imam affirms the words of the muezzin. However, there is no harm in starting the prayer after the iqamah is finished. In fact, according to Imam Abu Yusuf and the other three madhhab imams, this is preferable. (Ibn Abidin, Radd al-Muhtar, 1/479)

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu