blank GL class code in Sales order Creation

Hello All,

I am having a scenario wherein its needed to have blank GL class code during sales order entry and user will key in the GL class code by himself while creating the order. They use promotional offers to customer but would like to keep same order type with different GL class code to hit different revenue account based on type of promotional offers.

challenge here is that same product can be used in other order types also within same branch location but there GL class code should default form F41021 only.

I am wondering if there is a way to default a blank GL class code based on an order type/P4210 versions.