So i got and item called the *Staff of acid* https://www.dandwiki.com/wiki/Staff_of_Acid_(5e_Equipment).

Im a warlock with Pact of Chain (familiar) and i wanna cast *contagion* and deliver it through my familiar, however it seems that i cant do that.
either Because  my familiar would not be able to handle magic that isnt Mine.
or because im not casting the spell myself even tho the item says otherwise.

I dont really see and issue, nor a situation where the DM Choses how it works.

and would it happend with scrolls to?

Can anyone explain?
also heres the familiar spell.
https://roll20.net/compendium/dnd5e/Find%20Familiar#content