Open app in current directory when another app with the same name exists, from command line
In this case, open -a is NOT what you want; the -a option is for launching known applications (indexed in the Launch Services database that lets you open a document and get the right application) by name, not by pathname, or for opening files with a given application.

What you are looking for is just plain opening foo.app, just like you double-clicked it in the Finder. To do that,

open foo.app


This will open exactly that application, without involving the Launch Services database at all.