@HAPC , Thanks for posting in Q&A. Yes, your understanding is correct. Based on Google article, the Managed Google Play no longer supports the ability to purchase app licenses. Here is a link with more details:
https://support.google.com/googleplay/work/answer/6150398?hl=en
Note: Non-Microsoft link, just for the reference.
One way I can think is you can contact app developer to get the apk to install. After researching, I didn't find any more information about the paid app install. I think you can contact google support to get more help.
Thanks for your understanding.
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