Download the video:

In facebook, right click on the video → Copy video URL at current time
→ paste it in fbdown.net (for public videos), fbdown.net/private-downloader.php (for private videos)
→ remove ?t=NNN at the end → Download

For example, if you got

https://www.facebook.com/ubaid.hussain.507/videos/2580838562238346/?t=12

paste it and change it to

https://www.facebook.com/ubaid.hussain.507/videos/2580838562238346

Edit the video if necessary.

For example, to copy the first 10.5 seconds of src.mp4 to dest.mp4

ffmpeg -ss 00:00:00 -i src.mp4 -to 00:00:10.5 -c copy dest.mp4

Upload the video to whatsapp.