Trotter impressed in a three month loan spell at the end of the last campaign and he was offered a contract by Nigel Adkins before the United boss left for holiday early last week.
United would have to pay some sort of compensation for the midfielder as he is under the age of 24 and if terms can not be agreed between the two clubs then the valuation would be set by a tribunal.
This would be similar to the scenario that saw Scunthorpe fork out £335,000 to Stoke for Martin Paterson at the start of the previous Championship campaign.