如何与外商讨价还价？？？

基本上外国制造商的payment discount都是在以下几种情况：
1）提早付款：合同规定如4个月内付款，但是我方提早付款可要求贴现（cash in advance premium)
2）产品瑕疵
3）延迟交货

但从你的情形上来看，不知合同中规定的付款期限为何，所以我建议你用stock hand-over（库存压力移转的理由）来要求折扣，但是这应该还比较牵强，可是还能试试看：

Based on years' cooperations, we would like to pay your company for clearing all account receiveables and I do believe that's exactly what you prefer under such recession in the world. However, the stock pressure comes to us now and we are in charge of the following risk on sales/stock. Hence, could you please kindly consider for a certain precentage of premium on such favor we are giving? We are suggesting XX% as the discount and I look forward to having your kind confirmation.