These would have been good questions for the border officer. What "visa" did he suggest?
You were requesting entry on B1 business visitor
status (like 1000's of other businesspersons were doing that day), and certainly merely attending a meeting (since you are a contractor and not an employee) is certainly permitted.
What kind of "Letter" are you proposing to write? a
TN letter? You don't need a TN, thuis wages are immaterial to this proposed "letter".
If anything, your US client, on their letterhead, can write a letter to CBP which you will carry, explaining the purpose and duration of your trip, and the fact that you not their employee and that you reside in canada. You will still be requesting entry on B1
status, not any other "visa" .
That said, you *can* have your client write you a TN letter, but you need to be doing work that is covered by TN (that would cover what you normally do for them, not simply attending this meeting), and you need to be qualified for TN (you didn't say what kind of work you do). If such a letter were to be prepared, with a description of remuneration, then why on earth would you think it wise to write anything other than a truthful and accurate statement??
In any event, it sounds more like you got a grumpy officer. I would have pressed this up to the supervisor. The fact he used the word "visa" would seem to indicate that he was more intersted in scaring you off than properly doing his job (Cdns never get visas, they enter in a status). I guess it worked. It wouldn't be a bad idea to run this by a US immig lawyer familiar with NAFTA entry.