In Primavera p6, there is an option to track the cost but how do we track sales/ invoices?

Primavera is used for planning projects If you need sales / invoices, then I guess you should go for an accounting cum inventory module that can be linked to this program, which I haven't come across.

Primavera can help in the cost management of your project but is not adoptable directly to help in sales invoice management.

You can do it under notebook for an activity or use documents option.

for this you need to hacve and practice with SAP