Database
[{"_id":1,"partialAmount":"15,00"},{"_id":2,"partialAmount":"20,00"}]
Query
db.collection.aggregate([{$project:{_id:null,partialAmountArray:{$split:["$partialAmount",","]}}},{$project:{_id:null,partialAmountDotString:{$concat:[{$arrayElemAt:["$partialAmountArray",0]},".",{$arrayElemAt:["$partialAmountArray",1]}]}}},{$group:{_id:null,totalAmount:{$sum:{$toDouble:"$partialAmountDotString"}}}}])