Yes, they can be both "Really?!" in slang.
If I tell you '!صحيح', it could mean 'Really!' for exclamation. And it could mean 'Right' e.g. آه,صحيح. "Yes, right".
'والله!' is a swear word, it also could be used in exclamation or swearing.
والله in exclamation literally means "Swear by god!". In a simple sentence you use it in swearing as I told you e.g. والله لن أذهب. = I swear I won't go.
I hope you get it now. ;)