Middlesbrough have always felt that the German international was illegally approached by Gerard Houllier and they are seeking an extra £2million on top of the original transfer, and an extra £4million for losses projected from merchandise sales.
The High Court threw out the case in May but Lord Justice Simon Brown said the Teessiders should not be prevented from pursuing it further.
"It is all very well for Ziege to make categoric written statements of his thinking and intentions but this had not been tested by cross-examination," he said.
Despite granting Middlesbrough a further chance to state their case Brown made it clear they would not be guaranteed ultimate victory.
"Many assumptions have been made in favour of Middlesbrough. It may be that when the case is fully heard some will prove unfounded," he said.
"The claim is not bound to succeed but the club must be entitled to advance it."