How to solve null values being transformed (or not) for JSON post by the modelbinder
Try using a custom model binder, it may not ultimately be necessary but it will help you debug what is actually happening with the value passed to the server.
It is a very easy process and is really just two steps:
1) Create a class that implements the System.Web.Mvc.IModelBinder interface, I believe it is just the one method, BindModel().
2) Add the binder in the OnApplicationStart() method of the Global.asax.cs like so
ModelBinders.Binders.Add(typeof(AdjustmentDefaultsModel), new YourModelBinder());