The keeper currently plays for Norweigan side Lillestrom who are fighting for the championship and the cup and they wont let him come to Oakwell until a replacement is found according to Lillestrom's technical director, ex red Jan Aage Fjortoft.
Muller's contract doesnt run out until the end of the Norway season in November and Reds Chairman Gordon Shepherd told BBC Radio Sheffield:
"He's still a player in Norway, and until they sign another keeper I doubt that they will let Muller out.
"But we are talking to them about him constantly. I would like to see him here sooner rather than later."
It is beleived manager Simon Davey is also looking for another goalkeeper to challenge Nick Colgan next season.