I think the best short term solution is to contract foreign ATCOs at the relative rate and increase bonuses to the locals to the average equivalent salarys being paid to the leasees.
The thing is (my best guess..) that ATNS won't pay the leased individual's salary, he will continue to get the money from back home as before. ATNS probably pays the other ANSP a lump-sum for hiring X ATCOs. That sum would naturally be higher than just the normal salaries of the leasees.
But, Tesox, you're right about what should be the fairest way of doing it. (bonuses for the SA:s to level the differences)