Converting .NET (DotNet) System.Decimal to c/al Decimal

I have a property on a .NET Object that is of type decimal? (Nullable decimal), which i need as an entry in a table Field (decimal).
I created a variable of type System.Decimal to make the code more explisitt - and according to MSDN there is a implisitt mapping between System.Decimal (.NET) to Decimal (C/AL).

    Decimal := Invoice.Tax;
    IF NOT ISNULL(Decimal) THEN
      "Harvest Invoice Header".Tax := Decimal;

But this gives me the following error:
"Type Conversion is not possible because 1 of the operators contains an invalid type.

Decimal := DotNet"

Is there anyways around this?