Coupon Manager is fine for coupons issued by the manufacturer or your own generated coupons with a Fixed Dollar amount.
However, if the value of the coupon will fluctuate, create an inventory item with a "negative 1.00 default value" and "Ask For Price" option instead.
SETUP
1) Go to Inventory Center and choose ADD
2) Create a category named Coupons. This allows you to run reports on the use of the coupon. Depending upon your needs select Merchandise as the Tax Category which will reduce the amount of sales tax owed by the customer.
3) Enter a Description and SKU of your choice. Cost is 0.00 and selling prices are 0.00
4) On the Options [F6] tab UNCHECK "Track Stock" and "Allow Discounts" and CHECK "Ask For Price"
5) On the Defaults [F7] tab set the "Default Sell Quantity" to -1.00