From the British FCO Webpage of star_world's link:

"You must pay an airport departure tax of 25 Convertible Pesos (CUCs) per person when leaving Cuba. This must be paid in local currency."

This would constitute "spending money in Cuba" in violation of US law, esp. since it goes to a Cuban govt. agency.

Please note that 1 CUC is pegged as = $1.08, but US$ exchanged in Cuba are subject to a 10% penalty tax, so U will pay about $1.20 for a peso. This penalty may be avoided by bringing Canadian $, English pounds (no Scottish or N.I.) or Euros.