wrote:
Does someone have a definite answer to this question?

Player A and Player B are playing a blitz game for sizeable (at least
- to them) stakes.

Stakes are irrelevant.


Player A has the piece in his hand to make the mating move, when
Player B calls his tiime down (before he can complete the move). Who
wins?

Under FIDE rules, Player B wins on time.


An Eastern European spectator said that in "his country" you were
always allowed to "complete" the mate.

It is possible that his national federation uses a different rule set,
like the USA. (FIDE doesn't allow federations to use rule sets that
contradict FIDE's but the USCF has got away with this forever.)
Perhaps, though, he was confused about FIDE Article 5.1:

``The game is won by the player who has checkmated his opponent`s
king. This immediately ends the game, provided that the move
producing the checkmate position was a legal move.''

This means that, if player A had released the piece on the square that
delivers checkmate, the game is over and he has won, even if his flag
falls after that point. But if the flag falls before the move is
completed, A loses on time.


Dave.

--
David Richerby Pickled Widget (TM): it's like a
www.chiark.greenend.org.uk/~davidr/ thingy but it's preserved in vinegar!