I don't think it is a sin per se, since he gave it to you. If it is your personal item, you can do it as you like. This is how I understand it.
The question is why would you do it behind his back, as you put it. Do you have a good reason for this?
It might not me a sin, but it might be imprudent, since he might see this sell as a lack of affection on your part.