Is it permissible for a husband to have vaginal sex with his wife from her back
with or without an excuse, or is it impermissible?
It is permissible for the husband to have sexual intercourse with his wife from the back as long as they will have vaginal and not anal intercourse. Anal intercourse is unlawful in any way according to the saying of Allah (Exalted be He):
Your wives are a tilth for you, so go to your tilth, when or how you will, and send (good deeds, or ask Allâh to bestow upon you pious offspring) for your ownselves beforehand. And fear Allâh, and know that you are to meet Him (in the Hereafter), and give good tidings to the believers (O Muhammad صلى الله عليه وسلم)
The Prophet (peace be upon him) said:
He who has sexual intercourse with his wife through her anus is accursed.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.