You can't. After rescuing your sister from the Forsaken Fortress, you might notice that Mila and her father (the ones who WERE rich) are now poor, while Maggie and her father are rich. If you go out in front of the auction house at night, you'll see Mila standing there suspiciously. Talking to her results in her shooing you away; However, when you attempt to walk away, she starts walking down the road. This is a stealth segment (metal gear windfall?). If you get too close to her, she'll see you, and flee. If that happens, you'll have to wait until the next night or go into a building then come back out. What ends up happening is you follow Mila around town for a minute, ultimately ending up at the Eskimo's shop. When she attempts to open the safe (you'll know, there'll be stuff above her head indicating effort) you simply walk up to her and press A. Panicked, she tries to talk you into letting her go. Say no, and listen to her story. After she's done talking, she'll say her conscience feels clear, thanks you by giving you an Empty Bottle, and runs off.