Is this donation to the Obama campaign legal?

There are donation limits, and I think that it is pretty clear that a private citizen can't simply cover what are real campaign costs in an amount that exceeds what he is legally allowed to donate. From Fox News:

An anonymous donor has offered to pay up to $20,000 of the cost of extra security for President Obama's visit to a New Hampshire college town after the town asked the Obama campaign to foot the bill.
CBS Boston reports Jay Gooze, the Town Council Chair in Durham, announced Sunday a local resident has offered to cover the cost of the president's Monday campaign stop.
“We are grateful for this generous offer,” Gooze told CBS Boston.
Gooze says the resident stressed he or she believes the town did the right thing by asking the Obama campaign to pay up, and he or she felt donating the money is the right thing to do.
The town caused a controversy when they asked the Obama campaign to cover the $20,000 to $30,000 cost of extra police, fire and EMS services for his visit to the University of New Hampshire because the city couldn't afford it. The Obama for America campaign refused.
 Durham Town Manager Todd Selig tells CBS Boston residents had mixed reactions to the standoff. . . .

UPDATE: Compare it to Boston picking up Obama's security costs?  Will the city pick up the costs for Romney?  Will they do it for just one fund raiser or as many as Obama wants to make?  The Boston Herald has this discussion:

“It’s an atrocious waste of taxpayer money,” said David Tuerck, a government ethics watchdog with Suffolk University. “There is no taxpayer interest in any of this. It’s all about getting him re-elected, and the campaign should pay for everything.” . . .“(It’s) the president of the United States, and I respect the office he holds,” [Boston Mayor Thomas M. Menino] told WBZ-radio. “I’m not gonna let petty politics play any part of what the presidential visit (means) to the city of Boston.” . . . 

This last quote from Mayor Menino makes it sound as if Boston will only pick up Obama's security costs.