How to fix "The requested domain already exists in Azure database."

Question

I want to buy an app service domain I already had some years ago. I receive a conflict and the error message "The requested domain already exists in Azure database.", The domain itself is available. I had a different Azure account previously because of changes in the Partner Account I had to move everything to a completely different Azure tenant. So far everything worked except this domain. I went back to the old tenant, but there is this domain no more in. Seems it is in some internal database entry still existing, but I cannot remove it there.