We had some lumber delivered and paid for, but ended up not needing the size we had and our sales rep agreed to pick up what we did not need and drop off what we needed and we would just pay the difference, since the original drop was already paid for. The needed lumber was dropped, unneeded lumber picked up (we have pictures of return on truck) but we were never credited for the lumber picked up, only invoiced for drop. I did pay the difference between the 2 sizes. They have now filed a lien for balance.
You really should retain a construction attorney to review and evaluate your legal position based on the contract, communications, credits, and pertinent documents. The right to be entitled to file a mechanic's lien is set out in Texas Property Code Chapter 53. A construction attorney can evaluate whether or not the lien was properly and appropriately filed.
A supplier has no right to file a mechanic's lien if there is no debt owed. If the materials that are the subject of the lien have already been returned to the supplier, and there now is no debt, there is no right to file a lien.
Filing an invalid lien could subject the filer to liability under the Texas Fraudulent Lien Act, and the filer could be liable for statutory damages of $10,000, or actual damages, whichever is greater, plus attorney's fees.
Whether the proper notices were mailed to support a lien is another question for a construction attorney.
What kind of project (commercial or residential or homestead) is another issue to be evaluated by a construction attorney.
You should consider preparing a letter to mail by certified mail, return receipt requested, to the supplier to set out the facts and to demand the relief that you want. If the lien is invalid, state why. Make sure that your letter is professional and business like as it may be evidence later.
Good luck.