Abbott wants to sign the Essex club's forward Vernon in a move that will see Williams travel south.
But Colchester have yet to rubber-stamp the package and Abbott is threatening to call the deal off if nothing happens soon.
"I have put a bit of pressure on Colchester for the deal to happen," he told the Cumberland News. "We weren't too bothered at the start, because Ben has done terrifically well for us. But we think the deal as it stands is a good one for the club.
"If it hasn't gone through by the end of Friday, we might end up calling it off. That will enable us to do something else, which is based on work that has been done on the back of that deal happening."
Abbott could opt to move for Vernon in a separate deal as he needs to bolster his striking options after Danny Graham joined Watford on Thursday while Michael Bridges has turned down the club's latest contract offer.