Kozluk made only 17 appearances for the Blades last season

Kozluk, 29, who was the Blades' longest-serving player, turned down an offer from them and from Leeds United before deciding to move to Oakwell.

German-born Werling, 24, has joined from Turkish side Sakaryaspor.

Meanwhile, the Tykes have had a bid for Scunthorpe full-back Marcus Williams rejected, and have ended their interest in the 21-year-old.

"The clubs' valuation of the player is significantly different," a Barnsley statement read.

I can't believe how Sheffield United can pop back in with an offer two hours before I was to sign for Barnsley when we've had since Christmas last year to sort it out Rob Kozluk

Meanwhile, defender Robbie Williams, 22, has rejected Barnsley's offer of a new contract, and will remain at Oakwell on a week-to-week deal until his future is resolved.

However, as he is under 24, the Tykes are entitled to compensation if he moves on.

Explaining his move, Kozluk told BBC Radio Sheffield: "At Barnsley, (manager) Simon Davey was saying we've granted you the highest wage - that's how much we respect you.

"As regards playing, if I'm not injured I will be playing week in week out at Barnsley, whereas at Sheffield United I'm not so sure.