Hi,
I think this might be a standard feature.
Let us just think of a case here -> In case the Blocked Document is corrected, what do you think the FI Document Number is to be ? It should be the same document number that was reserved for the SD Billing Document[ you have referred this as SAP Billing Document. Right ?].
This is not Number Jumping as such.
Coming to legal issue, I do not have anything that I can say for that.
My two cents.
Thanks.