Hey there,
Currently, it’s not possible to create a “sale” for those who own a certain project.
There actually is a way to do that. When you are creating a sale, it’s possible to create a “Sale Condition”, like in the image shown below:
When you click the “Add Condition” box, you can have a condition that only users that own another project will be allowed to benefit from that sale.
Hope that’s what you were looking for :)