Maybe I missed it on the news, but how do we know about Russia offering a bounty on US troops?

It was in the presidents daily intelligence briefings. Now it was made public. 

You want the short or the long story... the short story the United States Intelligence Community found about Russia offering a bounty on US troops they also got a financial trail, but the long story... once a upon a time the USSR entered neighboring Afghanistan in 1979, attempting to shore up the newly-established pro-Soviet regime in Kabul. In short order, nearly 100,000 Soviet soldiers took control of major cities and highways. Rebellion was swift and broad, and the Soviets dealt harshly with the Mujahideen rebels and those who supported them, foreign support propped up the diverse group of rebels, pouring in from Iran, Pakistan, China, and the United States. The United States train and finance some Talibans a guy called Bin Laden appear and after the Afghan war end attack the US, the United States invade  Afghanistan after the September 11 attacks in late 2001... you can read about the war and ISIS later on 2020 at the 29 February, the United States and the Taliban signed a conditional peace deal in Doha, Qatar[95] that called for a prisoner exchange within ten days and was supposed to lead to U.S. troops withdrawal from Afghanistan within 14 months, however, the Afghan government was not a party to the deal, and, in a press conference the next day, criticized the deal for being "signed behind closed doors," between TRUMP government and Talibans. Iran and Russia, emboldened by their alliance in the Syrian Civil War, have also initiated a 'proxy war' in Afghanistan against the US.... a variety of sources, got an intelligence assessment that Russian agents were offering bounties to the Taliban for the killing of US or coalition troops had been around since March; that significant amounts of cash had been seized in US raids; and that some US personnel may indeed have been killed as a result.

It is fake news.  It did not happen.  Liberals are out there lying again.

It was made up by the media and eager to find another fake conspiracy; democrats said it was so.