Hi Hieke,

I wrote a little snippet for you that works

You could do this with a code-snippet plugin or add this in the theme child function.php file

Add the code, go to the blog-post-page, use quick-edit, add the word sold to labels and the badge will show "sold" instead of the date.

I hope this works for you!

Edit, i uploaded the code so it's easier to copy

/ Tobias