When a sales person creates an order in Shopify Admin, allow them to add a discount that only applies to certain customers but allows them to change the discount amount. For example, a discount called "Pro Custom" that only applies to "Pro" customers but the sales person can edit the percentage discount. This allows for reporting on the uses of the "Pro Custom" discount code without requiring every sales person to remember how to type in the name of the discount code (which you could do with a manual discount... but might lead to various typos that affect reporting).