Best Buy employee didn't know what he or she was talking about, sorry to say. One of the key features everyone who bought the original Prime (201) were cheering about when the Infinity (700) was announced was that the dock would still be compatible. It is, in fact, compatible. There was nothing wrong with your dock - it only begins charging the tablet back up once the tablet discharges a certain amount. It's a backup battery. Your tablet should be decreasing first. You can test this by charging them both up, taking note of what percentage they're at, and then turn on wifi, syncing, and brightness up. Anything to lower the battery. Leave and come back several hours later, perhaps a full day. Eventually you will see the keyboard dock's percentage really low, but the tablet's percentage is *somehow magically* actually higher or about where you left it before. This is because the keyboard dock began charging the tablet.
In the end, I hope it was just an exchange like you said, and no money transferred hands (besides the waste of gas and time), because it doesn't sound like there was anything wrong with your original dock.