ECC 6.0 Update Rebate Billing - VBOF (cannot update because Document num

I have a few questions about Rebate Processing in ECC 6.0

1.) Rebate Agreement Billing update (Document number 1000 9000#### 2010 was already assigned) error message in VBOF.

Steps? why?

2.) Why can we not created final settlement for Rebate Agreeement if Agreement Status is set to C - Settlement has been created is greyed out? We cannot update the Billing with VBOF also... Steps to remove C and then update?

3.) Can we cancel an existing credit memo request for a Rebate Agreement? Where can we find this?

Thank you for your answers.

SDK