How to configure Samba to use the password found in /etc/shadow instead of its own database, for one particular user?

Question

In /etc/samba/smb.conf, I already have the following set for ages:

unix password sync = yes

Yet, I have a user which his samba password is different than that found under /etc/shadow. I suspect that this happened because once, smbpasswd -u <user> was called to that user and since, Samba always check for the user's password in its own database instead of the one supplied by PAM (i.e. /etc/shadow).

So the question is, how do I instruct Samba to return to use Linux password for that particular user?

The questions applies to both Linux Ubuntu 16.04 LTS and 18.04 LTS.