I am fairly certain that he is lying about this

If we leave without a deal, we do not have to pay the £39 billion. That is the clear legal position as set out in the House of Lords report of March 2017. #BackBoris @BackBoris pic.twitter.com/tooClBp9Uf

— Jacob Rees-Mogg (@Jacob_Rees_Mogg) June 18, 2019