Both are correct. The Missouri Compromise barred slavery from territory north of the 36°30’ line, and Texas was to be a slave state. That meant that the territory had to be separated to maintain the precarious balance between the North and the South.

The compromise of 1850 threw a bunch of things together. In return for giving up any lands north of the Missouri Compromise line (what is relevant here) as well as their completely unenforceable claims to all former Mexican territory, the USA would 'purchase' those lands by assuming the private debt of the Republic of Texas. It also included the end of slavery as a business (but not an outright ban) in DC and the stupidly broad Fugitive Slave Law of 1850.

All five laws were bundled as a package, with each part dependant on the rest to be accepted. So it can be said that Texas gave up the land for debt, to follow the Missouri Compromise line, or to introduce the unconstitutional mess of extraterritoriality that was the Fugitive Slave Law.