Database

[{"_id":1,"feedId":2,"events":{"1500":{"t":0,"a":{"Manager":"Joe","Amount":20.3}},"1800":{"t":1,"a":{"Manager":"Joe","Amount":21.3}},"2100":{"t":3,"a":{"Manager":"Joe","Amount":22.3}},"2400":{"t":2,"a":{"Manager":"Joe","Amount":23.3}}}},{"_id":12,"feedId":2,"events":{"1200":{"t":4,"a":{"Manager":"Joe","Amount":24.3}},"1800":{"t":5,"a":{"Manager":"Joe","Amount":25.3}},"2400":{"t":7,"a":{"Manager":"Joe","Amount":26.3}},"2700":{"t":6,"a":{"Manager":"Joe","Amount":27.3}}}},]

Query

db.collection.aggregate([{$match:{feedId:2}},{$project:{events:{$objectToArray:"$events"}}},{"$unwind":"$events"},{"$sort":{"events.v.t":1}},{$group:{"_id":null,Amounts:{$push:"$events.v.a.Amount"}}},{"$project":{"Amounts":1,"_id":0}}])

Result