I think it depends on the terms of the engagement. If the firm has agreed to pass along charges for travel and incidentals "at cost", then the client is entitled to the rebate and it is dishonest for the firm to keep the rebate; I am not sure most clients ponder this too deeply, but I would think a typical client would assume that its consultants, if they work for large firms, have volume discounts and that cost is lower than full price.

But these kinds of things should really be covered in the engagement letter, and if the client wants to be assured of receiving the benefit of the volume discount(s), he/she should make sure that dimension is covered in the engagement letter itself.