[{id:1,branch:"a",amount:5,type:"sell"},{id:2,branch:"a",amount:9,type:"pass"},{id:3,branch:"a",amount:12,type:"credit"},{id:4,branch:"b",amount:2,type:"pass"},{id:5,branch:"b",amount:22,type:"pass"},{id:6,branch:"b",amount:6,type:"sell"}]

db.collection.aggregate([{"$group":{"_id":"$branch","doc":{"$push":{"type":"$type","amount":"$amount","branch":"$branch"}}}},{"$project":{"_id":{"$arrayElemAt":["$doc",{"$indexOfArray":["$doc.amount",{"$max":"$doc.amount"}]}]}}},{"$project":{"branch":"$_id.branch","type":"$_id.type","_id":0}}])