How shall you write a letter to your client to reject the request for discount for the goods you have already supplied to them?

A good business letter never says "no" even when the answer is no.

In business, goods/products have a listed price or a contracted price. The price of the goods should be known to the customer before they are shipped/sent. Having accepted the goods at the listed or contracted price, completes the suppliers end of the contract. The customer can't change a contract after receipt of goods.

Now, how to say this in a professional way.

Dear Sir:

Thank you or your order. I am in receipt of your request for a discount on the goods provided.

The invoice amount due is the listed (or negotiated) price at the time of sale (or placement of the order). At this time we have the ability to negotiate prices for future orders only. Please contact me before your next order so that we may set up a contract price that is suitable for both our needs.

I look forward to meeting your needs and working with you in the future.