db.test.aggregate([ {
      $match: {
         $expr: {
            $let: {
               vars: {
                  total: 100,
               },
               in: { $gt: [ "$total", "$$total" ] }
            }
         }
      }
   }]) 

db.test.aggregate([ {
      $match: {
         $expr: {
            $let: {
               vars: {
                  unused_var: 'unused_value',
               },
               in: { $gt: [ "$total", 100 ] }
            }
         }
      }
   }])