  Off-shell and relativistic bound-state corrections for the decay $t \to b+W$
are calculated to $O(\a_s^2)$ making full use of the Bethe-Salpeter formalism
for weakly bound systems. Thus we are able to take into account all terms to
that order in a systematic and straightforward manner. One of the previously
not considered contributions cancels precisely gauge dependent terms which
appeared in an earlier off-shell calculation. Important cancellations also
determine the gauge-independent part.
