Hello, oh man, now it’s getting exciting. i wanted to take a screenshot in firefox, that’s easier than in the app, and via firefox it went smoothly. The problem seems to be the app. I will try to reproduce with an other invoice
In the total amount field (which is blank) set it to 23.90. In the invoice amount field (which is 23.90) set it to 23.80. This tells the app to create a payment for 23.90 and to apply 23.80 to the invoice.