That's an interesting question!
However the others are correct.
You can't use Slash and Burns effect retroperspectively.
Let me compare the effect to Mongrel Man:
"Whenever another creature is destroyed, you may draw a card. "
'Whenever' seems to allow you to get the effect of Slash and Burn as well as of Mongrel Man. Imagine casting Volcana Charger and the use the new mana to pay for Mongrel Man. You could use it retroperspectively BUT you have payed for Mongrel Man (or Slash and Burns) effect AFTER you cast/payed for Volcana Charger. You can only making an effectiv valid and working if you tap mana for it and for that point on, you will receive the effect of Slash and Burn.
Maybe this article of the dm wikia can help you a bit:
"Casting a spell consists of the following procedure.
1) Reveal the card from its current zone.
2) If it has a Modal ability, choose it. (Hyperspatial spell, Double Evolution Burst)
3) If you have additional or Alternative Cost's, apply them.
4) If there is any Cost Increase or Decrease, apply it.
5) Pay the total cost of the card by tapping cards in your mana zone for mana,
After the above, resolve the spell." <-- this is the reason why you don't get slash and burns effect retroperspectively.
I hope i could help you.