Is it legal for someone to sale you a house knowing he have s a lend on it

This sounds like a contract-for-deed/ owner finance situation. There might be some pretty strong laws in place to protect you. Talk ASAP to a Real Estate attorney.

I agree you should consult with a local attorney familiar with real estate law. If it was a contract for deed the Texas legislature placed many restrictions on these types of agreements and you may have recourse against person that sold it (owner?). Consult with an attorney at your earliest opportunity. Do note that whom ever the legal owner is at this point he/she does not have the right to force you out and must go through the eviction process before he/she can remove you and take possession. He/she must serve you with 5 day notice so do know that is likely coming and you may have defenses and/or recourse you are unaware of without consulting with a knowledgeable attorney.