The Adelaide Exchange scheme was owned by Straben Developments. Tenants in the building include Belfast City Council.
The receivers report shows that the bank was acting with the US fund Lone Star which bought a portfolio of Ulster Bank loans in 2014.
The report also discloses that Straben are challenging the receivership with a court hearing due in September.
Straben is a joint venture between Stranmillis Investments, which is owned by James McGeown, and Benmore Developments (NI), which is owned by Kevin McKay.
The joint venture also owned Longbridge House in Belfast's Cathedral Quarter and it too is in receivership.
In May, Ulster Bank presented Straben with a demand to repay more than Â£28m and appointed the receivers shortly after.
The report states that a Lone Star company acquired "the economic interest" in the Ulster Bank loan in November 2014.
Lone Star is one of many US funds which have bought Northern Ireland property loans.
Earlier this year it sold a number of office buildings to the NI Executive.
A major office development in Belfast city centre has been placed into receivership by Ulster Bank.