Scenario:
A webmaster visits the links page of another site that is nothing to do with him -let's call it site 'A'. He writes to all their existing links offering a triangular link exchange, telling them that if they link to his site 'B', he will give them a link from site 'A'. In fact, to prove his honest intent he has already placed the link from site 'A'.
Response:
I have received a number of such requests. This is a clearly an attempt to fool Google -there should be only one response to such requests -immediately file a spam report at [google.com...]