aha ok.
so the coordinate system is reversed I guess. you have to apply correction to your y-value. If I understood correctly, it seems like the mid line is somehow corresponds to the center of the screen. If your x-value is good, then simply negating the y-value should solve the problem. Otherwise, may be you can try the following:
SFML- top-left = (0, 0). mouse position should be relative to this.
w, h = screen width, screen height
assuming your (0, 0) is (w/2, h/2). screen center = your camera center. (this is my assumption).
you can apply correction to your mouse_x and mouse_y
mouse_x = mouse_x - w/2
mouse_y = mouse_y - h/2
if your center is different you can apply correction to your received mouse coordinates accordingly.