ALEXANDRIA, Va., (AP)—An Alexandria engineer denied under oath Friday that he possesses a $12.5 million check two firms are seeking to attach as compensa tion for property seized by Cuba. Frank M. Austin said he had the check until mid-1959 when the two Cubans who had given it to him to purchase machinery de manded its return. “The Cubans became disenchan ted with Castro,” Austin declared. On the basis of Austin’s testi mony Judge Albert V. Bryan de nied a request for a temporary order which would have restrain ed Austin from disposing of the