payment and user.rb

class Payment
  include DataMapper::Resource

  property :id, Serial

  belongs_to :payee, 'User'
  belongs_to :payer, 'User'
end

class User
  include DataMapper::Resource

  property :id, Serial

  has n, :payments,   'Payment', :child_key => [:payer_id]
  has n, :receivings, 'Payment', :child_key => [:payee_id]

  has n, :payees, self, :through => :receivings, :via => :payee
  has n, :payers, self, :through => :payments, :via => :payer
end