You can call it a levy, a regulatory charge, or a recipe for making barbequed bananas, but if, in the end, whatever you are doing is actually a tax, then it is a tax, and will suffer whatever consequences flow from that.
The question of whether something is actually a tax or not comes up more often than you might think. It usually occurs when a province’s law or levy is in its result an indirect tax (which only the Federal Government can do) or when a levy or some other charge isn’t really doing anything except acting like a tax, instead of what it is called.
There’s a five-part test set out in the leading case of Lawson v. Interior Tree Fruit and Vegetable Committee of Direction,  S.C.R. 357 which has been faithfully followed in many other cases. But, apart from possibly making some helpful side comments about what taxes are and what they look like, they probably aren’t of much use. Most of those case are about some level of government making people pay certain (extra) amounts, as opposed to here, where the government isn’t giving the money in the first place.
That doesn’t mean it is impossible. Theoretically speaking, there is no reason to suppose that a take-away cannot be a tax merely because it is a “negative income” as opposed to a positive taking by the tax man.
Nonetheless, it is highly unlikely that any court would consider wage freezes a tax based solely on the fact that public employees are experiencing a negative income or some way of seeing it as a levy or a charge. It’s too broad, and would be wildly impractical because it would open governments to questions of whether something is actually a tax whenever they reduced salaries or tried to save money anywhere. You would need a reasonable limit on the idea, and a way to connect it directly to governmental taxation. It would end up being one of those case-by-case questions which depends on the context. So, let’s look at this one.
If any court was going to go for this, I think the most crucial factor would be how much of a link there is between the measure and the government’s ongoing tax policy. And in Manitoba, in the period of 2017-2019, the whole idea, justification, and rationale for the Public Services Sustainability Act is tied to the Province’s tax strategy.
This is actually most clear and explicit with respect to the teachers. As we have already seen, the government was bragging to Manitobans that their school property taxes won’t go up any more than 2% each year, and that the government can do that because they have [imposed a loss, or a charge, or a negative income, or whatever term some lawyer comes up with to refer to this type of “tax”] on the teachers which will save the school divisions all this money.
But you could also say similar things about the PSSA generally. At the same time that the Pallister Government is trying to save money on the salaries of all public employees, they have also been reducing personal income taxes as well as the PST. So, Government of Manitoba, what are you going to do about all that lost tax revenue? Well, they are going to replace it with the savings from the PSSA. But, if the sustainability savings from the PSSA are a replacement for tax revenue, aren’t they then, essentially, a source of tax revenue, and thus a tax, or equivalent to a tax?
It’s probably a high hurdle, but there is an argument to be made that the way the government went about the PSSA and why, given the context, what they were actually doing was substantially imposing an equivalent tax on the wages of public employees.