Spurs have been working hard on a swoop to snap up Porro and looked to have made significant progress, with a deal close, even though talks have left them frustrated.
It has been suggested that only the final details remain to be resolved and Porro will be able to make the move to north London.
However, according to Italian outlet Tutto Mercato, there has been a twist in the negotiations.
It is claimed that Porro’s €45m release clause is only valid in the summer and Sporting Lisbon now want more money in order to let him go.
It remains to be seen if Sporting Lisbon are asking for more money in order to let Porro make the switch to Spurs or if the deal remains on track.
Porro is a key target for Tottenham during this month’s transfer window, but Sporting Lisbon have been reluctant to lose him.
The Lisbon giants also have the Portuguese League Cup final this weekend and are keen that Porro plays in it.