What velocity data do you have? The CFD software will compute a velocity distribution across the outlet face. Do you have such a distribution or an "average" velocity? Typically, we like to use a velocity condition at the inlet or outlet and a pressure condition at other. Mathematically, this works best.

As you said, the temperature that is specified at the outlet is a backflow temperature. But this means that the value will only be used IF the flow is re-entering the domain at this point. If all the flow is leaving the domain at the outlet (typically what we want) then the backflow temperature is not used. The computed outlet temperature would come from the convected fluid upstream. If you specify the backflow temperature of 300K, this does NOT mean the solution must have a temperature of 300K at the exit.