Hi Yellowpoppy,
If the device is not able to be used to complete that transaction, regaining access to the Apple ID account will be the only way to disable the Find My service.
During the process to reset the password, you should be prompted with an option that you did not receive a code to pursue other means of recovery.
You may be able to use the steps from If you forgot your Apple ID password under "Reset your Apple ID password with the Apple Support app on a borrowed device":
"If you don't have an Apple device but have access to your trusted phone number, you can borrow an Apple device from a friend or family member, or use one at an Apple Store.
- Open the Apple Support app on the borrowed Apple device. If needed, you can download the Apple Support app from the App Store.
- Tap Passwords & Security > Reset Apple ID password > Get Started.
- Tap "A different Apple ID."
- Enter your Apple ID and follow the onscreen instructions to reset your password.
Any information that you enter will not be stored on the device."
Otherwise account recovery may be required: How to use account recovery when you can’t reset your Apple ID password.
Regards.