how can slash buy be changed as it is used by another plugin

Unless a plugin offers a configuration or lang file with that in it, you'd have to open the plugin in a text editor such as Notepad++ and change "buy" to whatever you'd like where the command is.

thank you for your answer, so I don't need to worry about the cs file just the language file correct?

Pretty sure this plugin would require you to edit the plugin.

At the moment you would have to edit the cs file. That said we have plans for a big update, including an in-game UI, push commands and other features, and a configurable buy command will be part of that!

well I tried editing it in the cs but it still overwrites the vehiclelicense mod that requires the /buy command  any suggestions?

I'd like to use this plugin but currently cannot reconfigure the /buy command to install it as that command is needed for another plugin.  Any way you can modify it for me with a command such as /vip?

Can't you just edit it in the  .cs file? 

I did, but it still states /buy command overwritten when reloaded

Then it obviously has it set somewhere else as well. Use Notepad++ and do a search through the files, or wait for the dev to update it.