A merits review means the Tribunal looks at all the circumstances around a decision and decides whether some other decision would have been preferable. It can then give an order substituting the preferable decision for the original one.
Usually, the Tribunal would not go directly to a hearing. Instead there would be a ‘preliminary conference’. The parties would front up and a Tribunal member would attempt to mediate a discussion about the issue. If the parties can be encouraged to reach some mutually agreed solution, then the Tribunal can give ‘consent orders’. So long as the solution would not actually be illegal, the Tribunal member won’t intervene in the mutually agreed solution, even if they think it was foolish for either or both parties to agree. The consent orders are then binding.
Another possibility is that a mediated conversation will lead the applicant to realise that their position is untenable and the orders may be to dismiss the matter.
Another possibility is that the parties might agree that the matter be put to a general meeting, since whatever a general meeting decides will over-ride the EC decision. Then the consent orders would be that the EC calls a meeting within some time frame and that the parties return to the Tribunal to report the outcome. If it goes against the applicant, the Tribunal might ask if they now accept the democratic decision of the OC. If they do, the matter will be dismissed. If they don’t, it might be set down for a hearing, this time with the applicant seeking an order to overturn the OC decision.
It might become clear at a preliminary conference that it would be beyond the powers of the EC to make the payment, even if they wanted to, because it is outside the realm of what would be defensible as consistent with an OC-approved budget. It might also become clear that the unit owner spent the money of their own volition at their own risk. The unit owner might argue that it was something the OC should have spent money on but I doubt that would count for much unless they could demonstrate that they had no choice but to spend the money (to avoid a greater cost or serious safety risk) because the EC was neglecting some responsibility even after the matter had been repeated brought to their attention.
It is hard to be more specific without knowing more of the specifics. As a general rule, an owner can’t just spend money and expect to be reimbursed without having that spending approved first.