[{"_id":1,"item":"sweatshirt","price.usd":45.99,qty:300},{"_id":2,"item":"winter coat","price.usd":499.99,qty:200},{"_id":3,"item":"sun dress","price.usd":199.99,qty:250}]

db.collection.aggregate([{$project:{rootAsArray:{"$objectToArray":"$$ROOT"}}},{$project:{newRoot:{$filter:{input:"$rootAsArray",as:"field",cond:{$eq:["$$field.k","price.usd"]}}}}},{$replaceRoot:{newRoot:{"$arrayToObject":"$newRoot"}}}])