[{payment:{sellersPayment:[{amount:50,isPaid:false,seller:1},{amount:80,isPaid:true,seller:2}]}}]

db.collection.aggregate([{$match:{"payment.sellersPayment":{$elemMatch:{seller:1,isPaid:false}}}},{$unwind:"$payment.sellersPayment"},{$match:{"payment.sellersPayment.seller":1,"payment.sellersPayment.isPaid":false}},{$group:{_id:null,confirmedBalance:{$sum:"$payment.sellersPayment.amount"}}},{$project:{_id:0,confirmedBalance:1}}])