Reverse Cleared Document

Dear Gurus,

I have the following issue:

Using FBR2, a new Vendor Invoice posting was created

Using F110, payment run was scheduled and posting was done for the vendor invoice

It was found that the Discount Base was wrong and the system had taken the value based on the refrence document.

Using OB32, I have made the field modifiable and have made sure that i have not choosen any stipulations.

Now when i tried to change, it is not possible as the area is greyed. I found out if there is a payment posting done it is not possible to make the change. Only option is to reverse the payment posting.

How to reverse a payment posting?/// How can i change the discount base? Please help me on this....

Thanks.

regards,

Raj