SQL Server never releases the memory it uses, unless told so by the operating system.

Your ERP module is not your SQL Server. When you close the application, SQL Server still runs in the background.

If this is not a server and you want to release memory back to the operating system, just cycle the SQL Server service. Since you say this is your production server, you don't really want to do that.

If you want to prevent SQL Server from using all your available RAM, set the Maximum Server Memory option in the instace properties. If this server is dedicated to SQL Server, you could set it at around 20 GB.