ここに全文あったわ
https://www.quora.com/What-does-the-letter-to-Robb-say-that-Theon-tore-up
Robb,

I hope this reaches you in time.
Father has rejected your offer and plans to attack the North, raiding the shores and taking Deepwood Motte.
Mobilize your army and make for the North before it’s too late. I’ll write again when I can.

Theon.