Hi,
you will need to substract the screen center to the mouse position, scale the resulting vector to your taste and place the camera in the point resulting from the sum of the player position and said vector.

Code:
camera->pan = 0;
camera->tilt = -90;
camera->roll = -90;
...
vecMouse.x = mouse_pos.x - ( screen_size.x / 2 );
vecMouse.y = ( screen_size.y / 2 ) - mouse_pos.y;
vecCameraOffset.x = player.x + vecMouse.x*0.8;
vecCameraOffset.y = player.y + vecMouse.y*0.8;
camera.x += (vecCameraOffset.x-camera.x) * time_step * 0.1;
camera.y += (vecCameraOffset.y-camera.y) * time_step * 0.1;


Salud!