Akaunting software is released under the GNU GPLv3 license from the Free Software Foundation. A copy of the license is also included with every copy of Akaunting, but you can also read the text of the license here. Part of this license outlines requirements for derivative works, such as apps.
May I user another license for non-PHP files?
May I sell apps?
Yes. However, you must distribute all of the files under the GNU GPLv3 license.