Credit note on investment

I received a credit note for an extra discount on a standing desk. I entered the original purchase as an investment, but when I try to enter the credit note under the investment category, I can’t save it.

Should a credit note on an investment be placed under a different category, or how does that work?

Of course, I could also change the original invoice to a regular expense instead of an investment. It’s about €300 original minus €120 from the credit note.