[{event:"Deposit",amount:10000000000000000000000},{event:"Withdraw",amount:50000000000000000000000}]

db.collection.aggregate([{"$match":{"$or":[{"event":"Deposit"},{"event":"Withdraw"}]}},{"$project":{"event":"$event","amount":{"$cond":{"if":{"$eq":["$event","Deposit"]},"then":"$amount","else":{"$multiply":[{$toDecimal:"$amount"},-1]}}}}}])