[{_id:"xyz",sales:[{_id:"aaa",quantity:3},{_id:"bbb",quantity:4}]}]

db.collection.aggregate([{$match:{_id:"xyz"}},{$project:{totalQuantity:{$sum:"$sales.quantity"},quantity:{$filter:{input:"$sales",cond:{$eq:["$$this._id","aaa"]}}}}},{$addFields:{quantity:{$sum:"$quantity.quantity"}}}])