TL;DR Uninstall app. Go to Play website. Login your preferred account. Request remote install of the app. In-app purchases can now be paid with your preferred account.
On my Android phone I use two Google accounts: my private and my work account. Both are linked to the Play store. While Android supports multiple accounts wonderfully in virtually all aspects, in-app purchases is the exception.
Whenever I wanted to use in-app purchases the Play store only offered me to pay through my work account. I don’t want to link purchases to my work account because I am certain that I won’t work for my employer forever. And I hate to pay twice.
Unfortunately, it is impossible to switch the accounts in the payment workflow or anywhere in the Android UI. The only solution I was able to find was to uninstall the app and request a remote install through the Play website with being logged-in in my private account.
As soon as the app is on your phone, you can pay the in-app purchase with the account you used to install the app.