Not sure why you would need to write code for this. The idea is to setup “Services” and “Products” in OpenERP, that the Cashier/Billing person can “add” to the patient’s quotation while creating a bill. So, if a person was admitted, then charges for bed can be added as a “service”.
For more information please read this:
User Manual for Stock Management: 3 - Stock Manager - Google Docs (See the section on how to create a service)
https://acespritechblog.com/2012/09/12/how-to-configure-product-in-openerp/