Sold A Undisclosed Lemon In Texas

I was sold a lemon car in Texas, after driving off the auto shop within 30 miles the car suddenly started overheating. I replaced the water pump, the coolant, and the thermostat and notified the dealer that I wanted a refund. The dealer negotiated with him saying they’d fix the car but I wanted nothing to do with them, they negotiated trying to let me select another car of their lot but I declined and wanted a full refund. I’m aware of the cars sold as is law in Texas but I was given a signed document by the auto shops agent AGREEING to give me a refund 30 days AFTER they’ve received the car back. I had it towed to them on the 22nd of last month so far I’ve received no refund and the 30 days is up on the 22nd of this month, the auto shop has avoided by calls from my phone number but conveniently answered from a different number and the agent stated the owner would call me back this upcoming Monday (currently Saturday) about the situation. I’ve read the google reviews and many people have had problems with this auto shop and the owner avoiding calls or always ‘conveniently out’ I had the car towed to them to 100% make sure they received the car as it had been 2 days and they made no effort to pick the car up. Am I in the clear and do I need to worry? As I have a SIGNED document stating they’d refund me