Inclusion in the value for GST u/s 15(2)(b)

Amount supplier is liable to pay, is actually paid by recipient, is not included in the price charged, but will form part of the value for GST. Example from a book : A sells goods to B for ₹ 10,000. It was A's obligation to transport goods to B's premises. But, B incurs ₹ 500 for such transport. So, B will pay only ₹ 9,500 to A after deducting freight charges. So, in this case, what is the value for GST in the hands of A. Is it ₹ 10,500 or ₹ 10,000? In either case, will it not be double taxation? Because, B would have already paid GST on freight charges of ₹ 500. If it is included again in A's hands as per 15 (2)(b), this amount would suffer tax two times right? Also, would B be eligible for ITC two times? This doesn't make sense. I believe value should only be ₹ 9,500 in A's hands. Could someone explain please?