Hi TW,
The OP has to come back and give us the details of the movement type used. We are not sure about the goods movement, and as the billing is done, then we assume that its a sale (stock moved out of SL) . Now, the stock could have moved from a restricted/blocked/safety stock etc..
If the invoice cancellation and reversal of GI is possible then its easy & good. If not then my suggestion may be considered regarding goods movement.
Regards
Jobi