Thanks for asking question! If I have understood right you have deleted your Azure App Service domain and wants to buy it again, If so you may want to know that If you have deleted your App Service Domain resource within the past 30 days, you can easily restore it by purchasing it again with the same Domain Registration Subscription and resource group.
Unlike trying to purchase the domain name from other subscriptions or resource groups, the validation will allow you to purchase the same domain name.
Check this blog Restore App Service domains within 30 days
Further If domain was deleted more than 30 days ago You cannot restore domain that got deleted more than 30 days ago, you have to wait for another 45 days after we delete at GoDaddy to repurchase the deleted domain in AppService Domain. If you wants to purchase the domain right away, then you can purchase it at GoDaddy by paying redemption fee.
https://my.godaddy.com/help/renewing-my-expired-domain-name-609
In case domain is deleted and also deleted Azure Subscription then you cannot restore these domains and now you have to wait 45 days to repurchase them in a different Azure subscription.
Hope this helps. Let us know if further query or issue remains.