A price of Material sold is Rs. 100, wherein 10% of the Tax is inclusive. then based on my calculation, the actual price of the material is Rs. 90 & Rs. 10 is tax.?

If we apply a tax of 10% on a Rs.90 product,

So,  90*10/100=9

10% of 90 is 9, you can even check it in a calculator.

So, total price of the product with GST becomes= Price of Product+ Tax
                                                                            = 90+9
                                                                            =99
It become 99 not 100.

Whereas when we check 10% of 90.9, it becomes 9.1

When we add 90.9 with 9.1, it becomes 100.