Going through Google Code Jam problem sets...

store_credit.rb

module Reader
  def read(lines_per_problem = 1)
    Enumerator.new do |yielder|
      gets.chomp.to_i.times do
        yielder.yield *[Array.new(lines_per_problem) { gets.chomp }]
      end
    end
  end
end

class StoreCredit
  extend Reader

  def self.start
    solutions = []
    read(3).each_with_index do |problem, index|
      solutions << "Case ##{index + 1}: #{solve(problem)}"
    end
    puts solutions
  end

  private

  def self.solve(problem)
    credit    = problem[0].to_i
    quantity  = problem[1].to_i
    items     = problem[2].split.map(&:to_i)

    inventory = {}
    items.each_with_index do |item, index|
      if inventory[item].nil?
        inventory[item] = index
      else
        inventory[-item] = index
      end
    end
    items.each do |item|
      complement = credit - item
      next if complement < 0
      complement *= -1 if complement == item
      if inventory[complement]
        pair = [inventory[item] + 1, inventory[complement] + 1].sort
        return "#{pair.min} #{pair.max}"
      end
    end
  end
end

StoreCredit.start