slavery was a tangled legal issue in Mexico and Texas. While out and out slavery was technically illegal, Mexican law did have 99-year indentured servant contracts, which was used to paper over some of the slaves in TX. However, it has to be realized that Mexican control over TX was so thin that quite a few slaves were there because the law was simply ignored. Santa Anna made a lot of noise about freeing the slaves in TX, but as far as I can tell, he freed only one; Travis' slave Joe (IIRC), whom he wanted to go up the road and spread panic about the Mexican force after the fall of the Alamo.
Of course, we're talking about Santa Anna winning the war here, so a lot of slaves would have been freed by default. SA was fairly brutal to the American expatriates he captured, massacring a lot of them. So if he had beaten Houston at San Jacinto and done the same thing to the captives, a lot of the slaves in TX wouldn't have owners anymore...