It would seem to me that you are mistaken. Santa Ana agreed to it per the Treaty of Velasco, which also had a provision requiring him to withdraw all Mexican troops to his side of the Rio Grande. Of course the Mexicans reneged on this almost immediately and renewed their efforts to reconquer Texas, but by that time Texas had already been welcomed into the community of nations whether Mexico liked it or not.
It is also false that the Mexicans acknowledged the Nueces River as their boundary with Texas. In fact they never even recognized Texas at all! In 1845 they still considered it a renegade province and were still trying to retake it. The Nueces River claim was nothing more than an arbitrary piece of specious nonsense they threw out after annexation in hopes of regaining the southern part of Texas since they knew they couldn't go up against the U.S. army. Thankfully General Taylor wasn't interested in playing their games.
And the Mexican's repudiated the agreement that Santa Anna had made.