Can a company forbid you from going to work for another company without a signed non compete?

Hi, my husband applied for a sales job with the sister company (company M) of his current employee (company S.) He was offered the position with Company M and signed the offer letter. Flights and hotel rooms have been booked for training. He does not have a non compete with his current employer and the two companies are both in the same industry but do not sell the same equipment. A few days after the offer letter was signed my husband receives a call that the president of Company M gave his word he would not "go after" employees from Company S and resended the offer. Evidently there is some history with sales people from Company S being approached by company M that my husband was unaware. The hiring manager and VP at company M were also unaware and pushed to keep the offer valid. Now the offer is valid if my husband can leave Company S on "good terms." My husband spoke with his current manager this morning and he said there is an agreement and he can't go work for Company M. Even though my husband applied for the job on his own accord and was not approached by company M. Now my husband is caught in the middle with a signed offer letter frim Comany M and no signed agreement with Company S or a copy of any agreement between company M and S. What options does he have?