建築師指令(Architect Instruction)

Your suggestion is only an ideal case and I think it seldom happens in reality.
Normally, time is a critical factor for construction works. If there is a variation, and we wait for contractor's quotation, and seek agreement and then Architect issues A.I., then this will avoid many arguments but in fact this will take a considerable period of time. If the project is held up due to this reason, it may come up with other consequences, e.g. delay in completion, deduction of LD, etc. Usually to avoid delay, contractors will do the work first and may also try to settle the variation at the same time.
For a normal contract, there should be contracual terms in guiding the valuation of variaiton, and usually assessed by a 3rd party (QS) to avoid any party demanding unreasonable price (either over or under estimate of the works) Whether with / without quotation is not a big problem since it will be faily assessed by QS. If both parties cannot reach an agreement, it may bring to arbitration / mediation for settlement.
Also, in usual practice, architect will be quite conservative in issuing instructions and they would rather issue instruction retrospectively. May be they do not want to make mistake so that until everything is clear or works have been done.

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