I sold my Apple Watch and i disconnected it but the buyer is saying that the watch is still on my Apple ID what do I need to do to remove it

From this link, this is what apple recommends --> What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

If you already gave away your Apple Watch, you can erase and turn off Activation Lock from iCloud.com. Here's how:

1. On a computer, go to Find My iPhone on iCloud.com and sign in with your Apple ID. 
2. Select All Devices, then click your Apple Watch.
3. Click Erase Apple Watch. Select Next until the device is erased.
4. Click the X button  next to your Apple Watch. 

Go to Settings > Apple ID (etc.). Scroll down and you'll see a list of your devices. Delete, if necessary.