There is an earlier GFII script (see link below) answering these questions. In this script, Tom was seriously considering the Houstan offer and his mistress (Sandra Corleone) tried to convince him to accept it a little before he entered the boathouse. In the boathouse scene, Michael thoroughly humiliated Tom by (i) showing that Al Neri's intelligence about Roth's whereabouts was superior than Tom's, (ii) showing that he knew about his affair with Sandra and (iii) questioning his loyalty. It was probably then that Tom decided not to accept the offer although he told Michael that he had already turned it down.


script