Well, I guess now that Trump has denied it, his supporters have permission to call the story fake news. What's it like to have to wait for the master to tell you what to think and what to say?
By the way, the story from Catherine Herridge at CBS cited by @Right_in_Virginia above quotes an anonymous source also. But I guess you believe that one because it's favorable to Trump.
Whether this story about bounties is true and whether Trump knew about it or not -- it certainly merits investigation. I am not going to take Trump's word or the word of the Russians or the Taliban that it never happened.
Russians flew planes against our pilots in Mig Alley, back in the Korean War.
Russians and Chinese advised and supplied the VC in Vietnam.
We have supplied a few combatants of our own, around the world.
As 'unsavory' as such things are, they occur.
When Russia (USSR) was fighting in Afghanistan, US supplied Stinger missiles (Shoulder launched, heat-seeking anti-aircraft missiles) brought down countless Soviet helicopters full of troops. They were used by the Mujahedin, who later became the Taliban. Seems they don't like anyone messing with their opium trade, among other things.
The only remedy, of course, is to catch them in the combat zone and kill them.
Even giving the report of bounties offered by Russia for killing coalition troops hypothetical credibility:
Did Trump have anything to do with bounties being offered?
Was he aware that bounties were being offered?
Could Trump stop those bounties from being offered?
What do you think would be an appropriate action?