If the price on the incoming invoice (purchase invoice) is higher than the previously agreed price, a debit note reflecting the price difference is sent by the buyer.
If the price on a purchase invoice is higher than it should be, you, as the customer, issue a sales invoice-price difference to your supplier.
An invoice creates a debt to accounts payable (your supplier).
Price Difference (Cost Reduction)
After the normal purchase transaction, if for any reason the seller and the customer agree on a price lower than the price on the purchase invoice, a debit invoice should be recorded reflecting the price difference. Since this discount reduces the cost, it is called a price difference (cost reduction).