You need a many-to-many relationship between sales and products. You would use an intermediary table, such as "salesitems" and form the following relationships:
Sales::PrimaryKey = SalesItems::Sales_ForeignKey
Products::PrimaryKey = SalesItems::Products_PrimaryKey
This type of thing is covered here:
This way you can show a portal of SalesItems records on either the sales layout or the products layout, and show related data from across the relationship.
You also need to update, FM11's file format is now out of date and will eventually fail to be supported. If you're developing a new file, you should be using the latest version.
I'd also recommend training, as this is basic RDBMS stuff that you should learn at a basic level. There are tons of FM authorized trainers, and online resources like lynda.com as well.