|Benedict & Maurus (left); Maurus & Placidus (right); God throughout|
Both were appropriately humble and ascribed the miracle to the other. Benedict naturally saw Maurus as the actor or instrument, since it was his feet that were pounding across the surface without actually sinking. Maurus countered that Benedict's order had been the trigger, empowering him to execute the miracle. He had not even been aware that he was performing a miracle; he was just executing an order that conveyed miraculous powers.
If I were judging the debate, I'd give Maurus some speaker points for creativity but the win goes to Benedict. Later monks, however, bypassed the question of whose miracle it was. Instead, they used this example to show that it is all right for a monk to argue with his abbot. Abbots were somewhat at a loss on that, since Benedict is virtually the final word for western monasticism. They concurred, but noted that Benedict and Maurus were outside the walls of the monastery, so the argument is limited to that.
Perhaps you are wondering how Placid got to be a saint? Maurus succeeded Benedict as abbot of Subiaco, but Placid accompanied Benedict to Monte Cassino (the Mother House) and continued to serve him, no doubt as the instrument of some miraculous demonstrations also.