Didn't understood your question properly. Do you want to say that deleted bill number (Eg. Bill Number 1) should be available for you to reuse it?
to accomplish your task you have following two options
1. use transaction objet, before generating your bill number if any customer return any item just get the ROLLBACK method
2. Add a 'return item' button in application, Accept item code and bill no on that form and when any customer return any item, search that item from specific billno and delete it from that bill, as always as add that item in stock again.
it is really simple
Editor, DotNetSpider MVM
Microsoft MVP 2014 [ASP.NET/IIS]
SRI RAMA PHANI BHUSHAN KAMBHAMPATI