The basis of this saying is from Roman days. It was once a common practice to offer roadside beggers from a variety of foods and alms, but the Senate, in 28 B.C.E., outlawed such generosity. Of course, this isn't true.

..spare any change?

Being in an obliging mood, my girlfriend dug out the contents of her pockets. 28 pence.

50p would be nice.

Being way too nice and perhaps a little stunned, she calmly explained it was all the change she had.

You might as well keep it then.

He walked off, doing something of a disservice to the genuinely needy everywhere.