No disrespect to your salesman, as I am one also, but he needs to get his facts straight on what YOUR SEER is. Why do you think its called an XL14? Why do you think the next model up is called and XL15...XL16 etc? With trane equipment that usually is the best the unit can do with vspd blower and proper coil. Why did he use a non-Trane product, your coil? Trane must have 15-20 different coils to match up with with furnace and ac.
To get your tax credit he needs to provide you with an SEER and AFUE fact sheet that list the indoor coil, ADP, and outdoor unit seer rating. Then and only then would your system qualify and your XL14 is not a qualifing 15 seer unit. You can submit anything you want to the irs and only if an audit is called would you get caught.
Please correct me if I am wrong on anything I stated above.
If this were me selling this job and I was mis-imformed about a seer rating and I told a customer he would qualify for a tax credit and the facts I gave were wrong, I think you should have the company you purchased this from either give you a qualifing unit or the tax credit they said you should get.