Please advise me regarding the sandals that should be worn for Umrah and Hajj?
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
The Prophet (Allah bless him and give him peace) said,
“He (a person in ihram) should not wear a shirt, nor pants, nor a turban, nor a cap, nor a gown. Nor [should he wear] khuffs unless he cannot find shoes, in which case he should cut them below the ka’b.” 
There is a difference of opinion regarding the connotation of the word ka’b. This has resulted in a difference of opinion in the style of the foot gear one should wear.
The ka’b is everything which protrudes. More specifically, it refers to the protruding bone on the ankle and the protruding bone on the upper foot.
The Maliki, Shafi’i and Hanbali schools are of the opinion that the word ka’b in the above hadith refers to the ankle. Hence, for those who adhere to these three schools, they must leave the ankle exposed.
The Hanafi scholars, in particular Imam Muhammad al-Hassan ash-Shaybaani ruled that the word ka’b refers to the instep on the foot. As the ankle and instep are both referred to as ka’b; to take the meaning of the instep in this issue will be greater precaution. Ihraam is a garment of humility which is reflected by dishevelment, untidiness and the exposure of the limbs. Precaution here would be to leave more of the foot exposed, as it coincides with the concept of Ihraam. Just as the face and head are left exposed, the feet should also be left exposed as much as possible. In addition, by taking the meaning of the instep, the ankle is also left exposed, so this opinion encompasses all opinions.
There is a lot of confusion regarding those straps on foot wear which go around the back end of the foot. Hadhrat Mufti Ahmed Khanpuri (Allah preserve him) has issued a ruling in Mahmud al-fatawa that it is not necessary to have the back end of the foot exposed. It will be permissible to wear sandals which have a strap covering the back end of the foot. However, the strap must not cover the area parallel to the ankle at the back of the foot. If the strap partially covers the area parallel to the ankle, then it will be overlooked. The main criteria are that the ankle and the instep remain exposed.
In conclusion, according to the Hanafi school of thought, only those sandals will be allowed to be worn in ihram which leave the instep exposed.
And Allah Ta’āla Knows Best
Mufti Faraz ibn Adam al-Mahmudi