Mongo conditionally sum values in project pipeline

use $filter before $sum

db.client.aggregate([
  {
    $lookup: {
      from: "invoices",
      localField: "_id",
      foreignField: "clientID",
      as: "invoices"
    }
  },
  {
    $set: {
      "invoices": {
        "$filter": {
          "input": "$invoices",
          "as": "i",
          "cond": { $gte: [ "$$i.total", 100 ] }
        }
      }
    }
  },
  {
    $project: {
      id: 1,
      taxID: 1,
      invoicesAmountGreaterThanOneHundred: {
        $sum: "$invoices.total"
      }
    }
  }
])

mongoplayground

use $reduce

db.client.aggregate([
  {
    $lookup: {
      from: "invoices",
      localField: "_id",
      foreignField: "clientID",
      as: "invoices"
    }
  },
  {
    $set: {
      "invoicesAmountGreaterThanOneHundred": {
        $reduce: {
          input: "$invoices",
          initialValue: "",
          in: {
            $sum: [
              "$$value",
              {
                $cond: {
                  if: { $gte: [ "$$this.total", 100 ] },
                  then: "$$this.total",
                  else: 0
                }
              }
            ]
          }
        }
      }
    }
  }
])

mongoplayground