Before a sensor can be used by another person or in a different account, it needs to be fully removed from your account. As long as the sensor is still registered to you, it can't be paired again.
Here's how:
Open the FYTA app and go to "My Devices"
Tap the sensor you want to pass on
Tap the three dots in the top right and select "Remove device"
Confirm the action
After that, the sensor is unlocked and the new owner can set it up in their own account by going through the normal setup process in the app.