Here is an opinion that will surprise:

"Many art historians identify Matthew with the bearded personage who with his hand seems to be pointing to himself, looking at Jesus. But more careful recent analysis of the painting has determined that this is one of the moneylenders -- with his other hand, he is giving money to one who is taking it -- and that his gesture is one of scandal, directed at the young man next to him. As if he were saying, 'Do you really mean him, the sinner, the unclean?'

A fuller explanation is here.