The version of events you're pushing doesn't make any sense. United didn't want to finalise the sale of Henderson before we signed Onana, that isn't the same as refusing to negotiate full stop. It's pretty simple so I don't know what's so hard to understand, but if Forest had presented an acceptable offer during that time, the Henderson deal would've gone through as soon as we signed Onana.
The deal is being held up because the two clubs can't agree terms, not because of some mythical start date United and Ten Hag have set for negotiations.