I've implemented OpenERP for a business to business operation but we also have a store open now. So I was told the Sales module doesn't affect warehouse inventory for cash sales and that I need to implement POS module.

What is the best way to implement this? Our cash sales are somewhat significant and entail the customer walking up to the salesperson, asking for a pre-sale which gets I put into the system and then the customer walks over to the cashier where the presale pops up in the system. The cashier turns it into an invoice and the client takes the paid invoice to the warehouse manager who hands him the goods.