Aircraft Technician reported lack of key information in A321 aircraft maintenance manual when removing a fuel pump lead to fuel spill.

Narrative

Aircraft X had an open discrepancy for the RH wing INBD fuel pump fuel leak. As the crew chief assigned with the aircraft; I requested a defuel of the aircraft because it had 22;000lbs total fuel. The Aircraft Maintenance Manual (AMM) does not call for a defuel but I decided it was best as a precaution. The aircraft was eventually defueled to 13;000 lbs total fuel but was imbalanced with fuel in the right wing tank. I instructed the mechanic to pump all of the fuel into the center tank which resulted in roughly 8000lbs of fuel in the center tank and the remaining fuel in the left wing. Once the fuel was moved; the mechanic began the removal procedure of the fuel pump in accordance with the AMM. All precautions and steps were followed. During the removal of the pump; the slide valve remained on the pump and was pulled from the wing tank as one unit. The slide valve was tightly sealed with O-rings on the fuel pump which prevented it from separating from the pump as it should have per the AMM. This caused a fuel leak from the wing. The mechanic immediately tried to stop the leak but was unable to successfully put the fuel pump back in place. He then contacted me by phone. I immediately went to the aircraft. I contacted management and was told that the fire department was already notified. Many mechanics; crew chiefs; and supervisors immediately got spill kits and attempted to contain the fuel. Mechanics were able to then separate the slide valve from the fuel pump on the ground and slide it into place on the wing to stop the leak. We then cleaned the spill.The slide valve is supposed to be locked in place by a flange. The flange is very small and easily worn; about the size of half a 3/16 washer. The pump was very sealed into the slide valve; so when it was removed; it came out as one assembly past the flange. In addition; the center fuel tank gravity fed fuel into the right wing which caused the large spill even after taking extra precautions to defuel the wing tank.Suggestions: There needs to be more details in the AMM as precautions to take when removing a fuel pump. The Aircraft Maintenance Manual does not suggest defueling the tank before removal of the pump. The Aircraft Maintenance Manual also does not caution in the removal procedures that on A321 aircraft the center fuel tank will gravity feed into the wing tanks if the center tank has less fuel. This spill could have been easily avoided if that precaution was present in the removal of the fuel pump reference.