The Prophet sallahu alayhi wa salam said:

إذا أفطر قال ذهب الظمأ وابتلت العروق وثبت الأجر إن شاء الله

He (Ibn Umar) said that the Prophet (ﷺ) said when he broke his fast: Thirst has gone, the arteries are moist, and the reward is sure, if Allah wills.

[Saheeh Sunan Abi Dawood no. 2357]
Imam al-Adheemabaadi rahimahullah said:

إذا أفطر ) أي بعد الإفطار

When he broke his fast: that is: after breaking the fast

[Awn al-Ma’bood]

Shaikh Salih al-Fawzan hafidhuhallah mentions that it should be read in summer and winter here

Shaikh Abdul-Muhsin al-‘Abbad hafidhuhallah was also asked:

Question:

The dua “The Thirst has gone, the arteries are moist…” and “Oh Allah I fasted for you, I believe in you, and with your provision I break my fast”.

When are they said: after breaking the fast or when breaking the fast?

Answer:

When breaking the fast. When you break your fast, you mention it. The authentic Hadith is the first one. As for the second one, it is not established

[Sharh Saheeh al-Bukhari no. 181 side B]
The second dua the Shaikh is referring to is the famous one most people use, but is not authentic:

اللهم لك صمت وبك آمنت وعلى رزقك أفطرت
So it seems that a person can say this dua before or after breaking the fast.

And Allah knows best

Translated by

Faisal Ibn Abdul Qaadir Ibn Hassan
Abu Sulaymaan