The FIA is set to reveal on Wednesday which teams have complied with last term's budget of 145million US dollars (£114m).
In the build-up to Sunday's Singapore Grand Prix, Red Bull faced unproven claims that they went beyond that figure, possibly by over five per cent, which would equate to more than USD7.25m (£5.6m). Red Bull deny the accusations.
Sanctions available to the FIA for a breach range from a reprimand, to financial penalties, to the deduction of drivers' points - and even exclusion from the championship.
Hamilton was denied a record eighth crown when he was beaten by Verstappen at a deeply contentious winner-takes-all finale in Abu Dhabi on December 12. Verstappen took the title by eight points.
Asked if he was nervous about the FIA's verdict, Horner said: "No. We're absolutely confident in our submission.
"Our audit was signed off by our auditors. We believe we are comfortably within the cap."
Mercedes boss Toto Wolff urged the FIA to take robust action against any team that has overspent. Horner responded by accusing Wolff of "hugely defamatory, fictitious claims" and threatened legal proceedings.
Asked if a lawsuit remained on the table, Horner said: "Of course. What I said yesterday absolutely still stands. We will consider all of our options.
"It was totally unacceptable to make a completely unfounded allegation - and on the basis of what knowledge? Where is this source of information?
"It was a confidential submission between the team and the FIA. I have no idea of the compliance of any of our rivals. So what does that information supposedly come from?
"We have not been informed that we are in breach so let's get to the end and see where we are."