Will I never be able to cook anything needing tomatoes in a seasoned cast iron Kadhai?
The PH scale ranges from 0 - 14 with the lower numbers being more acidic than the higher number. Canned tomatoes are generally around 3.5-4.5 on the acidic scale, in other words just above the half way point below neutral 7.

As long as your tomato based sauce is not left in your seasoned Kadai (as Kristina rightly points out) for a long timeframe and your kadhai is washed out immediately after use no damage to the seasoned coating should occur.

Just remember to wash out your Kadai (with warm water and a cloth) immediately after use to preserve the seasoned coating.