That's a problem affecting all negotiations. The powers are divided between the federal government and the provinces.
Parliament is free to impose obligations on the federal government to spend money. Clearly, the spending of money by the federal government in this area is under the finance power.
With respect to negotiations, each province is not necessarily treated the same with respect to funding, because the funding does take into account differences within the provinces--populations, legal aid costs, criminal justice statistics, etc. The funding formulas have changed from time to time, so if there were a legal obligation, there would still have to be some type of mechanism under the law for negotiations by the provinces and the federal government in order to provide a certain amount of flexibility. It would be possible to have a legal obligation, but there would still have to be some kind of flexibility to permit the negotiation that exists at present.