A few times when I was talking to some people in Mexico they used "manejar" instead of "tengo".
Did I understand them correctly and is this the case that "manejar" is used Mexico to mean "to have" sometimes?
According to the diccionario de americanismos:
III. 1. tr. Ho, RD. Tener algo, poseer. pop.
Mexico is not listed in the dictionary among those countries which use the expression, but that actually doesn't mean much. Probably the use of "manejar" as "tener" has become more popular in Mexico and the dictionary is still behind registering it.
You mean like something commented on this page, where you can find the phrase
"Le vengo manejando..."
That is used by sellers to emphasize what they are selling and differentiating it from what everyone else is able to offer you.
So the phrase le vengo manejando is used instead of today I have for you or something of the likes.
It is used alongside to have, you could interpret as "manage/managing"
Yo manejo cierta linea de productos
Can be stand as
I manage certain product line
I work with certain product line
Be aware of the context, because "manejar" also means to drive a machine/car/situation