Neither of those. Nobody would understand you if you said 'sadat' or 'sad thacha'. If you're familiar with phonemic script, this may help: /sed ðæt tə/

maybe it sounds more like "If you (sed)said that (tu)to someone."

You'd need to learn both "th" sounds properly (eg. as in "think" and "that"). Trying to replace them with another sound like "d" or "ch" is simply a mistake - you'll either say the wrong word, or at least say the word incorrectly. I agree that "sadat" and "sad thacha" make no sense. I wouldn't understand what you'd said either. Perhaps an audio example would be clearer? Have you tried saying the phrase on vocaroo.com? Then we could hear you.