It is allowed provided that you have no AA segments on the same ticket. If you attempt to use this ticket on an AA flight, it will be confiscated (required by current US law, so AA has no discretion in this matter). Whether or not you would be able to obtain a refund or replacement ticket is between you and the airline that issued the ticket. AA is prohibited from dealing with you as soon as you present the ticket, unless you have a waiver from the US State department allowing you to do commerce with Cuba (easier to get than you would imagine, but only for US citizens). Even though you are not leaving your flight in Cuba, it is paying landing fees there, and that qualifies it as "doing commerce with the enemy". BTW if you require a visa or visa waiver to enter the US, this would disqualify you for future US visits (unless the law is changed).