Employee and product relation.

My use case is as follow:

Let's say we have an employee whose function is "bodyguard".
Depending on his working time (late hours, week-end,...) his cost and sale price may vary.

I created different product for different working time.

eg:
Bodyguard Week day
Bodyguard Week night
Bodyguard Week-end Day
Bodyguard Week-end Night
...

I create a sale order that automatically creates task for the corresponding order lines.

Then I assign a project to the tasks and an employee to each task.

The employee can now put time in the task work field.

When I go and try to invoice tasks work, the product used for each tasks work line is the product related to the employee, not the product I used initially in the sale order.

I know its possible to force a product at the time you generate the invoice but I don't want to generate separated invoices by product !

What I want to do is having the task know the product it's related to base on the sale order and not on the employee executing the task.

I hope I make sense and someone can help me with this.

bests,
John.