A possible thing can be conceived as existing at different times and under different conditions. Specification (takhṣīṣ) means that one particular time, form, quantity, or occurrence is realized. Divine will is understood in relation to such determinate realization.
For instance, a tree might be conceived as coming into being today, tomorrow, or not at all. The mere abstract notion of possibility does not determine which state actually occurs.
Power relates to an outcome as possible; will relates to its specified realization. The eternality of both attributes is therefore not contradicted by the temporal existence of the tree.