How can I unlock a previously locked Apple Watch?

Some months ago I purchased an Apple Watch for the heart monitor function and after utilising it for intended purpose and thinking that I had in fact lost it not realising it was hidden in my house. I understand I must have locked it and then removed it from my device list having since found it forgetting that it was locked. I have sold it on marketplace only to be contacted yesterday after the purchaser gave it as a Christmas present to somebody and they’ve turned it on to find it. It is Locked.

I would like to sort this out for him. I’m really hoping people are gonna tell me it’s possible and not sucks to be you because I would like it not to be suck to me in this particular instance.

Help

[Re-Titled by Moderator]