if amount < 0
, then 0 - amount else
{ $project: { amount: { $cond: [ { $lt: ['$amount', 0] }, { $subtract: [0, '$amount'] }, '$amount' ] }}}
if amount < 0
, then 0 - amount else
{ $project: { amount: { $cond: [ { $lt: ['$amount', 0] }, { $subtract: [0, '$amount'] }, '$amount' ] }}}