Adjustment amount for item price is a negative value. For help fixing this, see http://sellercentral.amazon.co.uk/gp/help/25221
This error occurs when the Principal value in an adjustments feed (XML) is a negative value. If you uploaded a tab-delimited adjustments file, this error occurs when the item-price-adj value is a negative value. Principal and item-price-adj must be a positive value representing the refund amount you want applied to the Principal or item-price component of an order.
You cannot charge a customer's credit card by supplying a negative value. For more information on charging additional fees to a customer, please see the answer to How can I re-charge a customer's credit card?
To resolve this error resubmit your adjustment data with a positive Principal or item-price-adj value.