Transform third party API response in collection

Hello everyone,

I'm using a third party API to generate some billing information using a Job and now, I need to be able to concatenate the response of this API and some local data. Thus, I'm trying to achieve it using resources, but it is giving the following error: "Call to a member function first() on string" when I'm calling it as return BoletoResource::collection(json_decode($response->getBody(),true)['Billing']);. Could someone help me?

class BillingResource extends JsonResource
{
    /**
     * Transform the resource into an array.
     *
     * @param \Illuminate\Http\Request $request
     * @return array
     */
    public function toArray($request)
    {
        return [
            'IdBilling' => $this->resource['IdBoleto'],
            'DueDate' => $this->resource['DueDate'],
            'PaymentDate' => $this->resource['PaymentDate'],
            'CreditDate' => $this->resource['CreditDate'],
            'PrincipalValue' => $this->resource['PrincipalValue'],
            'InterestValue' => $this->resource['InterestValue'],
            'FineValue' => $this->resource['FineValue'],
            'TotalValue' => $this->resource['TotalValue'],
            'PaidValue' => $this->resource['PaidValue'],
            'Status' => $this->resource['Status'],
            'MailToSend' => $this->resource['MailToSend'],
            'PhoneToSend' => $this->resource['PhoneToSend'],
            'Comments' => $this->resource['Comments'],
            'OurNumber' => $this->resource['OurNumber'],
            'Identifier' => $this->resource['Identifier'],
            'ReturnMessage' => $this->resource['ReturnMessage'],
            'ReturnCode' => $this->resource['ReturnCode'],
            'Barcode' => $this->resource['Barcode'],
            'DigitableLine' => $this->resource['DigitableLine'],
            'OccurrenceMotive' => $this->resource['OccurrenceMotive'],
            'Url' => $this->resource['Url'],
            'CustomerName' => $this->resource['CustomerName'],
            'CustomerTaxNumber' => $this->resource['CustomerTaxNumber'],
            'PayerBankCode' => $this->resource['PayerBankCode'],
            'PayerBankBranchCode' => $this->resource['PayerBankBranchCode']
        ];
    }
}

class BillingCollection extends ResourceCollection
{
    /**
     * Transform the resource collection into an array.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return array
     */
    public function toArray($request)
    {
        return [
            'data' => $this->collection
                ->map
                ->toArray($request)
                ->all()
        ];
    }
}