The diagnosis is most likely hemolytic transfusion
The immediate step is to stop blood transfusion then
both recipient blood and the transfused blood should be checked for compatibility.
A sample of anti-coagulated blood from the recipient should be centrifuged
to detect free hemoglobin in the plasma. The patient should be hydrated
to prevent acute tubular necrosis.
N.B.: You may need to give mannitol to force diuresis
and it may help prevent renal damage.
You may need to give hydrocortisone with IV fluid in sever
cases (apprehension, dyspnea and hypotenssion)