Two things.
First, IIRC, buying hits is an optional rule. So it only applies if the GM allows it.
Second, you can only "buy hits" in non-stressful situations, when you can take as much time as you like to do the task at hand. I would not qualify setting off dynamite that you are holding as "non-stressful", no matter how the rules shake out in your favor.
Third, Shadowrun explosives have always been a tad weak. I had a player in 2nd edition who wanted to play a Troll that wore C12 strapped to his body and would act as a human bomb, because his body was maxxed out and he was abusing the armor layering rules to the point where he was nearly invulnerable. It's a side effect of trying to balance the game, unfortunately.
Fourth, don't use dynamite in Shadowrun. Always use grenades or C12. They pack a lot more punch.

Fifth... Ok, cracked SR4A. The base rating of Commercial Explosives is 3. You're factoring in a 1/4 of a kg of Dyanamite. I have no idea how much dynamite that translates into, but by Shadowrun terms, you're using only 1/4 of the normal minimum amount. So not much of a bang, for starters.
However, when you use explosives, you make a Demolitions plus Logic test, and add the number of hits to the DV. Might be an instance when you don't want to roll high! Plus, there's a couple ways I'd tackle someone being a smartass in my game and doing a stunt like this. One's by the book, and one's a bit of common sense.
The Common Sense way is simple. You're holding the dynamite and this was your intent. As such, I'd rule that all your dice automatically succeed. Which means you could be adding a LOT of DV to that one simple explosive.
The BTB way involves a bit of math, and lots of modifiers. Called Shot (+4 DV, or bypassing armor). Since you're not defending or trying to move away from it, maybe the attacker Prone Modifier (+3 Dice for the attack roll).
And then there's always the Chunky Salsa rules. Since the explosive power is so weak vs your Body rating, it's fair to say that the explosion isn't going to initially blow through the hand. Especially if you can resist the damage so easily. So unless you're delicately holding the Dynamite in the palm of you open hand, I could also rule the explosion rebounds. And rebounds. And rebounds. Sure, it loses -2 DV after it travels a meter, but you're closed hand is only a couple centimeters. I'd say it could easily rebound a couple dozen times, increasing the power each time.
The latter are all kinda dick moves, so I'm unlikely to ever use them, but... They all show ways that even a small amount of Dynamite can do a bit of damage. And in the amounts you're talking, sounds more like a Firecracker than a real explosive anyways. And I've had one of them go off in my hands. Stung like a bitch, my hand swelled a bit, and I lost some feeling in my fingers for a couple hours, but no real damage done.
Bull