You ' re right and wrong . To move a soul from your own gate is at the cost of crippling yourself as seen many times in the series . But this is ONLY because that gate exist within everyone and it still exist in their dimension . stated by Ed and teacher Pulling a soul from the otherside aka one that no longer exist has yet to be seen and is seemingly impossible . As there nothing you could trade from this world that would equal a human soul with life experiences that no longer exist . Also quoted by ed in the series In the end ed traded his knowledge of the truth and alchemy to gain back his brothers soul . This worked because his brother traded his soul like you said but the key to this was it was in Eds door of truth not als . Since ed and als doors were connected at the time Als soul wasn ' t lost just like his body like it would if been if they hadn ' t had that connection . If that connection wouldn ' t of existed Ed would be tall and al would be dead .