I think there's no literal drug deal. Bolton is just metaphorically talking about the quid pro quo. As CNBC puts that in context:
Former national security adviser John Bolton was so disturbed by the efforts to get the Ukrainians to investigate President Donald Trump’s political opponents that he called it a “drug deal,” former White House official Fiona Hill reportedly told Congress on Monday.
Bolton is known for his metaphorical language on more than one occasion relating to this matter. As that news story continues:
Hill, the former top Europe expert in Trump’s White House, testified that Bolton told her he wanted no part of the effort that involved acting White House chief of staff Mick Mulvaney, a person in the room for Hill’s testimony told NBC News. Bolton also was said to have referred to Rudy Giuliani as a “hand grenade.”
(And, for what's worth it, Giuliani called Bolton in return "an atomic bomb".)
Also note that Bolton himself hasn't testified and he probably won't.
But Bolton will not testify until the completion of a lawsuit filed in October asking a judge to rule on whether witnesses should abide by the testimony requests of lawmakers or White House directives not to testify.
That lawsuit was brought by Bolton’s former deputy Charles Kupperman after he was subpoenaed by lawmakers. The White House argued Kupperman could not be compelled to testify, claiming he had “constitutional immunity.”
The future of the lawsuit is uncertain given that the leaders of the inquiry have withdrawn Kupperman’s subpoena, and have asked the judge who was to hear the case to dismiss it. House leaders also chose not to subpoena Bolton after he missed his scheduled testimony Thursday. Responding to the letter from Cooper, Democratic lawmakers argued the suit would slow the pace of the inquiry unnecessarily, and suggested the White House’s claim of immunity could become an article of impeachment.