How to save a record with a form in a modal with related tables in Laravel?

I'm trying to save a record in a modal. But in that record there is a relationship. I have a code that supposedly mind does. When I register the information, I do not get an error, but it does not save my record.

The function is as follows: in the account index it shows me the account information among them the clients with their account details but it has a button where it says to credit the account since it is a credit. Then the function is that I go updating the balance of the account for the amount entered from the modal to pay.

These are the relationships of the tables:

In the index of the account I have a button that passes the id of the account to the modal:

     <a data-target="#modal-abonar-{{$cue->idcuenta}}" data-toggle="modal" href="">
                            <button class="btn btn-success">
                                Abonar
                            </button>
     </a>

This is the modal file (create) where I record the information:

<div class="modal fade modal-slide-in-right" role="dialog" aria-hidden="true" tabindex="-1" id="modal-abonar-{{$cue->idcuenta}}">

    <div class="modal-dialog">
        <div class="modal-content">
            <div class="modal-header">
                <button type="button" class="close" data-dismiss="modal" aria-label="Close">
                    <span aria-hidden="true">x</span>
                </button>
                <h4 class="modal-title" id="myModalLabel">¿Deseas agregar un abono a la cuenta?</h4>
            </div>
            <div class="modal-body">

                {!! Form::open(array('action'=>array('AbonoController@abonar',$cue->idcuenta), 'method'=>'POST','autocomplete'=>'off'))!!}
                {{Form::token()}}
                <div class="row">
                    <div class="col-lg-6 col-sm-6 col-md-6 col-xs-12">
                        <div class="form-group">
                            <label for="cliente">
                                Cliente
                            </label>
                            <input class="form-control" name="cliente" value="{{$cue->cliente}}" disabled>
                        </div>
                    </div>
                    <div class="col-lg-6 col-sm-6 col-md-6 col-xs-12">
                        <div class="form-group">
                            <label for="cantidad">
                                Cantidad
                            </label>
                            <input class="form-control" name="cantidad" placeholder="cantidad.." type="number" min="0">
                        </div>
                    </div>

                    <div class="col-lg-6 col-sm-6 col-md-6 col-xs-12">
                        <div class="form-group">
                            <label for="fecha">
                                    Fecha
                                </label>
                            <input class="form-control" name="fecha" placeholder="fecha.." value="<?php echo date(" Y-m-d ");?>">
                        </div>
                    </div>

                    <div class="col-lg-6 col-sm-6 col-md-6 col-xs-12">
                        <div class="form-group">
                            <label for="observacion">
                                Observacion
                            </label>
                            <!--si el texto esta validado muestra el observacion ingresado-->
                            <input class="form-control" name="observacion" placeholder="observacion.." type="text">
                        </div>
                    </div>
                </div>
            </div>
            <div class="modal-footer">
                <button type="button" class="btn btn-default" data-dismiss="modal">Cerrar</button>
                <button type="submit" class="btn btn-primary" data-dismiss="modal">Agregar</button>
            </div>
        </div>
    </div>
    {{Form::close()}}
</div>

In the subscription controller I have the function that registers the subscription (SubscriptionController):

public function abonar($id, Request $request)
    {
    //se busca el campo saldo del id correspondiente de la tabla cuenta
    $sqlcuenta = Cuenta::findOrFail($id);
    //  $sqlcuenta = Cuenta::where('idcuenta', '=', $id);
    $saldo     = $sqlcuenta->saldo;

    //se resta el saldo con la cantidad ingresada del formulario
    $cantidad = $request->get('cantidad');
    $saldo    = $saldo - $cantidad;

    //se actualiza el campo saldo de la cuenta con el id correspondiente
    $cuentaSaldo =Cuenta::findOrFail($id);
    //  ->update(['saldo' => $saldo]);
    $cuentaSaldo->saldo=$saldo;

    $cuentaSaldo->save();

    //-------SE BUSCA LA ULTIMA CUOTA PARA INCREMENTARLA-----
    $noCuota = Abono::select('no_cuota')
    ->where('cuenta_id', '=', $id)
    ->orderBy('idabono', 'desc')
    ->limit(1)
    ->get();

    if ($noCuota) {

    $noCuota = $noCuota->no_cuota;

    } else {
    $noCuota = 0;
    }
    $noCuota = $noCuota + 1;

    //---------------------SE INSERTA EL ABONO---------

    $abono                   = new Abono();
    $abono->cuenta_id       = $request->get('cliente');
    $abono->cantidad = $request->get('cantidad');
    $abono->fecha  = $request->get('fecha');
    $abono->observacion             = $request->get('observacion');
    $abono->no_cuota=$noCuota;
    $abono->saldo_abono=$cuentaSaldo;

    return Redirect::to('cuentas/abono');
    }

This is the route (web):

Route::post('cuentas/abono','AbonoController@abonar');