What you can do here
A unit cannot be given to a patient it was not found compatible for
Hanging a bag is refused unless there is a crossmatch for that exact unit and that exact patient saying compatible. The check happens when you press the button, so a result recorded as incompatible a moment ago still stops you.
A crossmatch that is not back is not a pass
Blood is not given on the assumption that a test will come back clear, so a pending result refuses the transfusion exactly as an incompatible one does.
This does not decide compatibility
It records what the laboratory found. Nothing reads a result off a pair of blood groups, because a crossmatch finds antibodies a group does not predict, and a system that guessed would be wrong in the cases that matter.
One unit number, one unit
Booking the same number in twice is refused, because a duplicate is two records of one bag and neither is right about where it is.
A unit issued for somebody else cannot be hung
The patient it was issued for is checked against the patient in front of you, at the moment you press the button.
Expiry is checked when the bag is hung
A unit is usable through its expiry date, not up to it, and a unit past it is refused.