Because when they said 'equivalent exchange', I thought, oh, Yuuko made scientific. Written in a rush.
The harming of the 'body in the physical plane, the fate in the astrological realm, and the soul in the spiritual plane' (or something along those lines, at any rate) is taken from one of the earlier chapters in xXxHoLic, and one of the final ones in Tsubasa. Tried to incorporate Watanuki's growth; starting to understand the balance of prices and why self-sacrifice is no good, but not quite ready yet to apply that to himself. Oh, you silly fool.
Disclaimer: FMA is a masterpiece that I can only dream of creating. And CLAMP, just CLAMP.
"Give him back!"
Watanuki nearly drops the tray, so startled is he by the sudden scream. Yuuko looks up, and something flickers in her dark eyes, but it disappears quickly enough that he wonders if he hadn't imagined it.
"Yuuko-san...?" It is not the first time the shop is catering to long-distance customers, but to only hear the voice of this one is unusual and unnerving. Watanuki is starting to understand the magnitude of need one requires to even enter Yuuko's shop, and the raw desperation and pain in those words is enough to move his hand to his right eye in sympathetic pain, for the price this customer will undoubtedly have to pay for his wish.
"He's my brother! Take my leg, my arm, my heart, anything!"
"No!" The tray slams against the wooden table, sake sloshing out of the bottle, as Watanuki turns to Yuuko. "The price, it's too much! Yuuko-san, tell him-!"
"There is no need."
"But, the customer, he'll be hurt! Like you said, in body, soul and fate!"
"Edward Elric's world is based on equivalent exchange." She pours a glass of sake for herself from the half-spilt bottle and downs it like a shot. "He understands what price needs to be paid in exchange for that which he values most."
"You can have it! Just give him back! He's my little brother, he's all I have left!"
"It is more of a toll that he pays, and not to me, but to a different power altogether. My favourite kind of customer; he has determination and the drive to move forward under his own power. No, Fullmetal has no need of my services."
The emergency dealt with, Watanuki finally registers the sake splattered all over his hands and the table. Yelping, he pulls out a dishcloth from his apron pocket and starts wiping everything down.
"...Will he get his little brother back?"
His eyes are focused on his task, so he cannot see the warm smile that appears on his employer's face. "Yes. He does."