db={"properties":[{"_id":ObjectId("551816b02eecf1238b3baadb"),"saleNumber":NumberInt(17917),"saleDetails":{"amountOwed":266.0,}},{"_id":ObjectId("551816b02eecf1238b3baadc"),"saleNumber":NumberInt(851400070),"saleDetails":{"amountOwed":270.0,}}],"newProperties":[{"_id":ObjectId("551816b02eecf1238b3baadb"),"saleNumber":NumberInt(17917),"saleDetails":{"amountOwed":300.0,}}]}

db.properties.aggregate([{"$lookup":{"from":"newProperties","localField":"saleNumber","foreignField":"saleNumber","as":"newPropertiesLookup"}},{/** only matched records are kept*/"$unwind":"$newPropertiesLookup"},{$project:{"saleDetails.amountOwed":"$newPropertiesLookup.saleDetails.amountOwed"}},{"$merge":{"into":"properties","on":"_id","whenMatched":"merge"}}])