CARTAGENA, Colombia — A Secret Service agent preparing for President Obama’s arrival at an international summit meeting and a single mother from Colombia who makes a living as a high-priced escort faced off in a room at the Hotel Caribe a week ago over how much he owed her for the previous night’s intercourse. “I tell him, ‘Baby, my cash money,’ ” the woman said in her first public comments on a spat that would soon spiral into a full-blown scandal. The dispute — he offered $30 for services she thought they had agreed were worth 25 times that —...