The way things work here is a doctors note/PERSCRIPTION trumps Probation Officer.

Let's take this to extreme. You are on probation. Get in a car wreck, Get hurt real bad. Opiates are given to you. Next test is dirty. Will they chuck you in jail? No. A doctor perscribed those meds to you.

Same thing with MMJ.

Now cover your ass. Talk to a lawyer. Will cost you a bit but then you will know where you stand. I would bet you will be able to tell your PO to go pound sand.