How to serialize Decimal Fields in mongoengine marshmallow?

I have a decimal field in mongoengine, and I am trying to return the data I have stored in the model, but when I try to return the object I am receiving an error in the encoding

TypeError: Object of type 'Decimal' is not JSON serializable

class PaymentTransactionSchema(Document):
    meta = {
        'collection': 'payment_transaction'
    }
    payment_mode = StringField()
    payment_reference_code = StringField()

    currency = StringField()
    amount = DecimalField()
    status = StringField()

    approval_code = StringField()
    remarks = StringField()

    createdate = DateTimeField(default=datetime.datetime.utcnow)


class ProjectFundTransaction(ModelSchema):
    class Meta:
        model = ProjectFundTransactionSchema

then a return like this

self.__transaction = PaymentTransactionSchema(**args)
res = self.__transaction.save()
transaction = self.dump(res).data

I was hoping that the marshmallow would handle this automatically but it seems that I missed a config somewhere