Presale item txns handling

[Leonard Poon]

I'm developing an inventory system for a company. I come across a practical
issue that I don't know how to handle the pre-sale item transactions in the
system. There are some items that are sold prior to their purchasing. This
means the sales invoice is inputted before its purchase invoice. I just need
some advice on the common way of handling this type of transaction in
computer. Any books suggested to me?

Thanks,

Leonard