The former Middlesbrough and Bury forward, who turned 21 earlier this month, was brought in by the Reds for a six-figure fee at the end of January - believed to be around for £250,000 plus add-ons - before immediately being loaned straight back out to Yorkshire rivals Bradford City for the remainder of 2018-19.
Miller had spent the first half of last season on loan at Valley Parade after joining from Boro - and agreed a three-and-a-half-year-deal with Barnsley following some complicated negotiations with the Bantams and the Teessiders.
Viewed as a long-term signing with the future in mind, Miller made his Reds' debut when he came on as a late substitute in the Championship opener with Fulham on August 3.
He then came off the bench and was handed 15 minutes of second-half action as a replacement in the recent Carabao Cup home loss to Carlisle United.
Miller netted six times for Bradford last season, having made his name at Bury.
The striker is behind the likes of Cauley Woodrow, Conor Chaplin, Mamadou Thiam, Mallik Wilks and Patrick Schmidt in the Oakwell pecking order, with the decision to loan him to the Iron taken in order to further aid his future development.