Example of use case:
Employees can work on more than one project, each project has a cost center associated with it.
When payroll is settled, it associates all employee timesheets to a single cost center (since only one cost center can be chosen when processing payroll).
As much as you filter by project, all timesheets for all projects are settled.
There are two possible solutions.
If you choose a project in the payroll process, in payroll slip just fetch the timesheets of that project, so they can be associated with their cost center.
Another option is to bring all the timesheets as you do now but not to use the payroll cost center process but to take the cost center of the time records.