Modifier to adjust the quantity of an item

Hi Pricing Experts!

I'm lookinf for a modifier to adjust the quantity of particular item.

Here a short example:

For the Order type "express job" I want to adjust an item for the labour value from 1 to 2 hours.

If I use an additional item as a promotion product I have to define a lot of different modifiers, because the labour values are customized for nearly every customer by different modifiers and the promotion product gets insert with a fixed price.

I want to adjust the quantity of the item only and the customer specific unit price of the item should be unaffected.