Hi Lokesh
First check if you can assign the same sort key to the GL account involved in the billing. Ideally it should be possible to update the sales order in assignment field that ways
If that does not work, you can write a substitution in OBBH and use user exit as a last option.
I think in SD also you have an option to influence what is populated in the text fields of FI doc
Br. Ajay M