I'm not sure what state you're in and how the purchase contracts are written, but if you were my client in my state I would be suspicious of the fact that this leak and "repair" was not disclosed, especially if you had requested due diligence documents. As the other people in this thread mentioned you will probably have to pay for the repair out of pocket since the warranty doesn't cover it. However my advice would be this, talk to your realtor and have them lean on the selling agent and mention that this wasn't disclosed and is illegal not to disclose known faults within a property. The sellers (and possibly agent who could also get in trouble for not disclosing if they knew) clearly knew about this issue since they gave you an invoice, which incriminates them. It's not huge enough that it would be worth going to court, but maybe your agent can get them to agree to give you some sort of compensation towards fixing the A/C. Also, as others mentioned I would get multiple quotes to see how much it will be to get replaced. You might be able to work with the warranty company some as well, sometimes they are a little more flexible than they lead on, and the squeaky wheel gets the grease. I understand the frustration and hope you get some sort of solution, best of luck!