problem in cancellation of invoice

dear gurus,

I had generated one billing document suppose 12345, after that I had cancelled that also (no: 99900), now suppose I will go again to cancel 12345, system is giving the error message u201Cinvoice is already cancelledu201D.

As per my understanding in background system checking vbrk-fksto field, so I changed that field again to black from u201CXu201D, even though system not allowing.

So kindly tell me what to do to solve this.

Regards

Amit