"But administration sources told Fox News that the reason for the freeze on Ukrainian aid was tied to concerns about corruption. Sources said that Trump wanted to be confident that the incoming administration of Volodymyr Zelensky was going to take steps to end corruption before releasing the funding.
The aid to Ukraine also, according to sources, came up in the context of other countries and questions about whether they were paying their share.
The White House’s decision to release the funding ultimately was made in mid-August, after Sen. Rob Portman, R-Ohio, called the president and said that Ukraine desperately needed the aid, according to administration sources.
The White House, at the time, was also up against an appropriation “impoundment” deadline, which meant the money would either have to be spent, or impounded. The White House would have been forced to go to court to justify the impoundment, which sources said it did not want to do."
https://www.foxnews.com/politics/trump-ukraine-aid-was-stalled-over-corruption-concerns-decries-impeachment-frenzy