"As the defenders of the Alamo were about to sacrifice their lives, other Texans were making clear the goals of the sacrifice at a constitutional convention for the new republic they hoped to create. In Section 9 of the General Provisions of the Constitution of the Republic of Texas, it is stated how the new republic would resolve their greatest problem under Mexican rule: All persons of color who were slaves for life previous to their emigration to Texas, and who are now held in bondage, shall remain in the like state of servitude ... Congress shall pass no laws to prohibit emigrants from bringing their slaves into the republic with them, and holding them by the same tenure by which such slaves were held in the United States; nor shall congress have power to emancipate slaves.
Mexico had in fact abolished slavery in 1829, causing panic among the Texas slaveholders, overwhelmingly immigrants from the south of the United States. They in turn sent Stephen Austin to Mexico City to complain. Austin was able to wrest from the Mexican authorities an exemption for the department -- Texas was technically a department of the state of Coahuila y Tejas -- that would allow the vile institution to continue. But it was an exemption reluctantly given, mainly because the authorities wanted to avoid rebellion in Texas when they already had problems in Yucatán and Guatemala. All of the leaders of Mexico, in itself only an independent country since 1821, were personally opposed to slavery, in part because of the influence of emissaries from the freed slave republic of Haiti. The exemption was, in their minds, a temporary measure and Texas slaveholders knew that."
Source: https://www.escapefromtexas.com/)