MongoDB projection. Operator $add field|expression array awareness or after $slice

Question

I've got collection that looks like:

[{
    "org": "A",
    "type": "simple",
    "payFor": 3,
    "price": 100
},
{
    "org": "A",
    "type": "custom",
    "payFor": 2,
    "price": 115
},
{
    "org": "B",
    "type": "simple",
    "payFor": 1,
    "price": 110
},
{
    "org": "B",
    "type": "custom",
    "payFor": 2,
    "price": 200
},
{
    "org": "B",
    "type": "custom",
    "payFor": 4,
    "price": 220
}]

And need to produce result with query to perform group by "org" where payments appears for only first "payFor" prices in "type". I'm trying to use expression result by $slice operator in $add but this is not works.

pipeline:

[{
    "$group": {
        "_id": {
            "org": "$org",
            "type": "$type"
        },
        "payFor": {
            "$max": "$payFor"
        },
        "count": {
            "$sum": 1
        },
        "prices": {
            "$push": "$price"
        }
    }
},
{
    "$group": {
        "_id": "$_id.org",
        "payments": {
            "$push": {
                "type": "$_id.type",
                "forFirst": "$payFor",
                "sum": {
                    "$cond": [
                        {
                            "$gte": [
                                "$payFor",
                                "$count"
                            ]
                        },
                        {
                            "$add": {
                                "$prices": {
                                    "$slice": "$count"
                                }
                            }
                        },
                        {
                            "$add": "$prices"
                        }
                    ]
                }
            }
        }
    }
}]

I know that it is possible to traverse unwinded prices and pick only "payFor" count of them. but result collections are more rich than in example above and this operation will produce some unecessary overheads.

Need some advice from community. Please. Thanks.