Sounds like your using Pricelists. Using only these, you wont be able to get the described result.
You can however, when enabling Promotions, Loyalty & Gift Card at Sales -> Configuration and configure a rule similar to the following:
Go to Products -> Discount & Loyalty and create a new Program:
Program Type: Promotions

Rules & Rewards-tab:
Conditional rules:
Minimum Quantity: 1
Minimum Purchase: 0
Grant: 1 Promo point(s)
Product Domain: Match all of the following rules: Product Category = X or Y

Rewards:
Reward Type: Discount
Discount: 50% on Specific Product
Discount Product Domain: Match all of the following rules: Include archived Product Category = X or Y
In exchange of: 20 Promo Point(s)

Your Sales Orders (or POS or Quotes created via Website) would then result in something like this:
Reward button wont do anything (in the manual scenario of a Sale Order) since the Rule applies to 20+ items of Clothes or Food in my example:

With required quantity:

Or with more items in total matching the Program:
