Link Timesheet to Invoice

We have contractors and consultants working in our system:

• They are assigned tasks on projects
• They enter hours against those tasks and submit time sheets.
• They also submit invoices for their time and expenses.
• It would be great if we could add a field to the timesheet for an Invoice number. Extra points if that just automatically updated the project with an invoice for their hours and expenses - and do it in a way where we could report on those invoices and include invoices from external entities that aren't working in our system (these would be normal invoices that are applied to projects)