Try:
sudo busybox passwd XY
(XY is your user name)
I have used the root privilege to set a weak password for a normal user, with a warning, but it worked in the end.
But I tried again recently and it failed.
My final solution is to use busybox's internal passwd program, which is a different program, and it works.
My system is gentoo, and after checking, the default passwd is from sys-apps/shadow-4.8.1-r2.
duke@duke-pc ~ $ sudo passwd duke
You can now choose the new password or passphrase.
A good password should be a mix of upper and lower case letters,
digits, and other characters. You can use a password
that consists of 8 characters.
A passphrase should be of at least 3 words, 8 to 40 characters
long, and contain enough different characters.
Alternatively, if no one else can see your terminal now, you can
pick this as your password: "sleep=Medal2brim".
Enter new password:
Weak password: too short.
Try again.
(....)
You can now choose the new password or passphrase.
A good password should be a mix of upper and lower case letters,
digits, and other characters. You can use a password
that consists of 8 characters.
A passphrase should be of at least 3 words, 8 to 40 characters
long, and contain enough different characters.
Alternatively, if no one else can see your terminal now, you can
pick this as your password: "Freeze+six7nazi".
Enter new password:
Weak password: based on a dictionary word and not a passphrase.
passwd: Authentication token manipulation error
passwd: password unchanged
duke@duke-pc ~ $
duke@duke-pc ~ $ sudo busybox passwd duke
Changing password for duke
New password:
Bad password: too weak
Retype password:
passwd: password for duke changed by root
duke@duke-pc ~ $