[{"status":"PARTIALLY_REFUNDED","amount":100,"FIELD_B":20,"FIELD_A":10},{"status":"PARTIALLY_REFUNDED","FIELD_B":20,"FIELD_A":10},{"status":"PARTIALLY_REFUNDED","amount":100,"FIELD_B":34,"FIELD_A":2},{"status":"PARTIALLY_REFUNDED","amount":100,"FIELD_B":30,"FIELD_A":22},{"status":"PARTIALLY_REFUNDED","FIELD_B":36,"FIELD_A":29},{"status":"PARTIALLY_REFUNDED","amount":100,"FIELD_B":355,"FIELD_A":23},]

db.collection.update({"status":"PARTIALLY_REFUNDED","amount":{"$exists":true}},[{"$set":{"amountRefundRemaining":{"$subtract":["$FIELD_B","$FIELD_A"]}}}],{"multi":true})