[{"BillingId":1,"TotalBilling":10,"Installments":[{"InstallmentId":"Inst01","TotalPaid":2},{"InstallmentId":"Inst02","TotalPaid":5}]}]

db.collection.aggregate([{"$addFields":{"TotalInstallments":5.0,"TotalBalanceDue":{"$subtract":["$TotalBilling","$TotalInstallments"]}}}])