It is not very clear why you want to do a left join. I have assumed that it is because there are invoices that simply do not have assigned movements, so the following fragment of code revolves around that.

var q = from fac in db.Facturas
        join mov in db.MovimientosDineros
            on fac.IDFactura equals mov.IdFactura into facturasMovimientos // 1
        from facMov in facturasMovimientos.DefaultIfEmpty() // 2
        group facMov by fac into grupo // 3
        let sum = grupo.Sum(g => g == null ? 0 : g.Valor) // 4
        select new 
        {
            IDFactura = grupo.key.IDFactura,
            Totalaportado = sum
        };

I will explain the code using the numbers I have indicated in the comments:

• In 1 what has been done is to join the invoices with their movements and has "englobado" this union in a table invoices Movements.
• In 2 we specify that if an invoice has no movements, a default value (null) is returned. Consult the documentation of this function if you see it necessary.

  

Summary: Returns the elements of the specified sequence or the   default value of the type parameter in a singleton collection if the > sequence is empty.

• In 3 we group all rows of invoices Movements by invoices.
• In 4 the total sum is calculated specifying that if any row is null the summand is 0.

Explained the code, I have to say that I have not been able to prove it, so if it contains any errors or my concepts are wrong, indicate it and I will correct it, please.

It could be something like

var cosnutla = from fac in db.Facturas
                join mov in db.MovimientosDineros on fac.IDFactura equals mov.IdFactura into g
                select new {
                    factura = fac,
                    Monto = g.Sum(x=>x.Valor)
                };

There are several ways to perform a left join, you could only group by the union

Perform grouped joins

or you may also apply conditions

Perform left outer joins

You can try something like that with lambda expressions:

var q = Facturas.Select(x =>
            new {
                Id = x.IDFactura,
                Sum = MovimientosDineros.Where(y => y.IDFactura == x.IDFactura).Sum(z => z.Valor)
            }
        ).ToList();

We simply go through the table of Invoices and for each one of them we will generate a new object, which will have as id the table id invoices and a field called Sum, which will contain the sum of all the values of the table MovementsMoney match the Id.

and if you create a class with a decorator, you fill the objects from sql in a list (with your inner join) and then the list works with linq ex:

 listado.sum(x => x.MovimientosDineros).Tolist();