As far as we know there is no hadith regarding the dua of sawm before sehri. Yes, there are both types of duas for iftar time dua are mentioned in hadith. The Ulama have explained that the Prophet (pbuh) used to read اللهم لك صمت الخ before iftar and ذهب الظمأ الخ after iftar.
Allah knows Best!
Darul Uloom Deoband