Originally Posted by
kirkwoodj
You'll be fine, as you're not selling or conducting any other business transaction
No bigger "business transaction" than proposing marriage...
Originally Posted by
bensyd
Goods over $900 need to be declared. There will be no taxes to pay as the OP plans to take the item back out of the country. Coming from Canada he's very unlikely to have his bags scanned.
If the ring was bought in on her finger then there would be no expectation that it would need to be declared. Technically you're probably correct that it should also be declared in this case, but I doubt there's a customs official out there that would take any offence at it not being declared.
(I'm presuming that the OP will have a return ticket out of the country, and is on a tourist visa/ETA - for a resident it would potentially be a different story)