Is the husband sinful if he prevents his wife from wearing a coat instead of the long abaya, even though she has a dispensation for that from some muftis?

You must obey your husband regarding wearing what covers your body, like the abaya, even if you believe that covering can be achieved without it.

If your husband does not permit you to go out wearing a coat (balto) while you believe it is permissible, it appears that he would not be sinful if he allowed you to wear it. This is based on the opinion of some scholars, such as a view held by the Shafi'is and Hanbalis, who state that a man should not prevent his wife from drinking a small amount of nabidh (fermented drink) if she believes it is permissible.

Muslims are advised to strive to reside in Muslim countries due to the dangers that living in disbelieving lands poses to one's religion and morals.