If the Islanders trade Andrew MacDonald, it’s because of money. A month a half ago, the Islanders defenseman turned down a four-year, $16 million offer, The Post has learned. MacDonald and his agent had previously asked for a four- or five-year deal at $5 million per, and even got the Islanders to come up from their original starting price of $3.5 million per. In turning down the offer, the pending unrestricted free agent has left himself as one of the league’s most highly coveted trade pieces, before both Friday’s 3 p.m. Olympic roster freeze and before the March 5 trade deadline. A league source said general manager Garth Snow already has received at least one substantial trade offer, and has turned it down. Assuming the market will only rise over the next 24 hours, as well as during the period after the two-week break and before the deadline, Snow is apparently banking on getting a better return for MacDonald than what he has been offered. Snow is believed to be looking for at least a first-round pick in return for the 27-year-old, who has played his whole career for the Islanders since being a sixth-round draft pick in 2006. Struggling as of late, MacDonald had a bit of a rebound performance in Washington on Tuesday, scoring the game-winning goal in a 1-0 defeat of the Capitals. In the five games before that, he had been a combined minus-8.