In order to request financial relief for the leave without pay that you incurred, you could:
pursue formal recourse where you would need to demonstrate that the employer acted inappropriately in applying the policy given the specific circumstances - through the filing of a grievance
pursue formal recourse where you would need to demonstrate that the employer acted in a discriminatory fashion in applying the policy given the specific circumstances- through a human rights complaint (Homepage | Canadian Human Rights Commission); or
through participating in a class action (see below).
Feds For Freedom Class Action – specifically for federal employees impacted by COVID-19 mandates:
https://sheikhlaw.ca/
National Class Action – focused on lockdowns and mandates, open to anyone in Canada:
https://gwsllp.ca/covid-19-class-actions/
Alberta Provincial Class Action – for those who experienced vaccine-related injuries:
https://rathandcompany.com/covid-19-vaccine-class-action/
These cases are seeking certification. If any of them seem applicable, it may be worth reaching out to the listed firms to ask if you can be added as a claimant once the class action is certified.
Generally, class actions are structured so that there is no upfront cost to claimants—lawyers work on a contingency basis and are paid through a percentage of the awarded compensation, subject to court approval. If the case doesn’t succeed, claimants typically don’t owe anything.