According to authentic ahadith, the Holy Prophet (SAW) used to recite two duas for iftar:

1) اللَهُمَّ لك صُمْتُ، وعلى رِزْقِكَ أفْطَرْتُ
(Abu Dawood, Babul Qaul ind al Iftar, Hadith No. 2360)
This dua is to be recited before iftar even though the past tense has been used in this dua. The use of the past tense in duas and other situations is part of the Arabic language convention.

2) ذَهَبَ الظَّمَأُ، وَابْتَلَّتِ الْعُرُوقُ، وَثَبَتَ الْأَجْرُ إِنْ شَاءَ اللہ
(Abu Dawood, Babul Qaul ind al Iftar, Hadith No. 2360)
This dua is to be recited after iftar.

Note: It has become very common for people to recite
للهم لك صمت وبك آمنت وعليك توكلت وعلى رزقك أفطرت
This version of the dua cannot be found in the hadith, therefore, the version that has been narrated in the hadith (اللَهُمَّ لك صُمْتُ، وعلى رِزْقِكَ أفْطَرْتُ) should be recited.