=== modified file 'account_payment/account_payment.py'
--- account_payment/account_payment.py	2013-08-30 11:07:16 +0000
+++ account_payment/account_payment.py	2013-08-30 11:13:10 +0000
@@ -532,6 +532,7 @@
     def onchange_move_line(self, cr, uid, ids, move_line_id, payment_type, date_prefered, date_scheduled, currency=False, company_currency=False, context=None):
         data = {}
         move_line_obj = self.pool.get('account.move.line')

         data['amount_currency'] = data['communication'] = data['partner_id'] = data['bank_id'] = data['amount'] = False


         if move_line_id:
             line = move_line_obj.browse(cr, uid, move_line_id, context=context)
-            data['amount_currency'] = line.amount_to_pay
+            if payment_type!='receivable':
+                data['amount_currency'] = line.amount_to_pay
+            else:
+                data['amount_currency'] = -line.amount_to_pay

             res = self.onchange_amount(cr, uid, ids, data['amount_currency'], currency, company_currency, context)