Not quite the same as your situation, but my Dad just went through surgery on account of his heart (and smoking cigarettes since he was 10). Ended up getting put in a wheelchair because of a pinched nerve and lack of blood flow during the surgery.

Recently he asked me to buy him smokes behind my Mom's back (she has done a lot of work to help him get back on his feet, and to keep him alive. Been by his side through it all). I tired to tell him no, but he claimed it helped him with depression issues.... which I get, continued smoking of cigs has helped me through quitting weed's ups and downs.....

Unfortunately, I don't have an answer to you question. It is messed up that your Dad doesn't seem to be taking your feelings into account, but then how often have we done that to our parents growing up? If it's that big of an issue, just say "Sorry, my connection dried up" (I wish I could pull that card on my Dad in our situation). What could he say? Is your Dad the type to flip out when he doesn't get what he wants?.... :S

I had a similar situation. Long story short: my dad and his second wife enabled me to smoke (a little bit) when I was a late teen - even gave me a pipe. But then after I dropped out of college and ended up in rehab in at 20 yrs, they acted like I was the worse thing ever and how dare I ever get into smoking weed, and I've ruined my life etc. Tons of judgement and negativity around the weed. Then flash forward another 15 years. My life was together (for the most part. Married, business, so forth) and my dad visits and he's an alcoholic and while drunk keeps asking for weed. Right? I had some weed, and was smoking regularly, but I had negative views on my smoking and didn't want to go there with my dad, so I didn't bring it out or admit I had any (I also didn't want to deal with his back-and-forth enabling and then later judgement on it). And now another 5 years later I'm trying to quit weed again (am 4 months sober today) and on the phone just last night my dad is hinting that it would be nice for me to have a vacation and just smoke weed. Previous to 4 months ago we were evicted from our apartment over a minor amount of $ and my dad at the time even got scolding, telling me to get my shit together. And that eviction was part of my inspiration this time around to quit smoking and drinking.
So, as you can see, there has been back and forth, back and forth. I don't even want to have anything to do with mixing substance abuse with family enabling or judgements.

I am in my early 20s too, been smoking for a decade. I have only smoked with my Dad a few times, but he would get it for me when ever I asked, always had to pay, but it was always there. I think he figured it was better that I get it through a trusted source rather than smoking laced shit. It is odd that your father would keep asking you for it if he knows you're trying to quit. This is pure speculation, but it sounds like your dad has his own addiction or problems that he needs to deal with and he shouldn't be taking his problems out on you.