Cancelling a Half-Paid Invoice That Was Already Refunded

[Quirk-kun]

Hello! I'm trying to cancel a sent invoice that was half paid and later refunded, but I'm having trouble figuring out how, since I keep getting error messages and can't find articles on them or my situation. Please see the (hopefully correctly spoiler'd) screenshots and captions included below. 

Spoiler

Tried to enter the amount that was refunded, then this popped up. Text under error message reads: "Refunded all of 50% amount ($10.50) paid on Jan 28, 2017, 4:14 PM"Unable to cancel because there was a partial payment, though the amount was refunded on Jan. 28.

Please walk me through how to cancel an invoice in this particular scenario. Any tips are appreciated. 

Thanks for your time! 

[Quirk-kun]

(Bumping; please help me if you have any advice!) 

[sharpiemarker]

Add a line in the invoice for the amount paid but with a minus sign to zero out the invoice?