Katy could empty that account at any time That would be a pretty lame virtual bank then :) Realistically, the spending conditions are either limited by time.

Katy could empty that account at any time That would be a pretty lame virtual bank then :) Realistically, the spending conditions are either limited by time. So Billie knows she needs to withdraw until a certain date or Katy could betray her. Or the spending conditions contain a punishment for betrayal. So if Katy tries to withdraw money she assigned to Billie, Billie can intervene, get her money and punish Katy.