N0m
1) “I understand them the same. Maybe I’m wrong. ”
If you understand Ein mevatlin issur lechatchila and davar sheysh lo matirin as the same (which I suspect you are mixing up) then you are wrong. They are completely different ideas.
2) sources for what?
If you want sources for Davar sheysh lo matirin its its own siman in Shulchn Aruch YD 102
If you want sources for Ein mevatlin issur lechatchila its a few seifim in YD 99: 5-6
Again they are completely different ideas