Hello @edith_t, Welcome back the the MongoDB community forum,
You can use $toInt or $toDecimal operator to convert type something like this,
db.Compras.aggregate([
{
$group: {
_id: { Agente: "$comercio.agente.idAgente" },
total: { $sum: { $toDecimal: "$compra.montoTotal" } }
}
}
]);