Lets not act like everything is on a known scale. The guy bought something he was told was different than the one he tried before (if I understand correct). At this point the dispensary did the appropriate thing and as such should not be asked in a manner fitting that they the dispensary did something wrong. I would suspect that they would want to make you happy. Its medicine not a street corner dime bag. I can also understand if they do not want the product back but on such a little amount I would think its possible they do something for you if your appropriate about it.
I see it akin to going to a restaurant and ordering something new and not liking but it is made properly, its just your taste that you do not like it. To you its not so good but to others it very well may be a favored dish. Both are views have merit and subjectivity but not necessarily warranted to be taken off the bill unless that is typical for that establishment.
To me it sounds like you bought a cheaper named herb, was told it was not the type you knew but you did it anyways and now want someone else to change your situation as you dislike your choice.
To me, I would accept that I took a risk but its up to you on what to do. Their is no wrong answer really. Just seems a bit cheap for a poor choice. Not meant ignorantly, just a perspective.