pass module args as extra variable

Hi,
in order to debug a playbook , I need to pass some value to a module_arg at run time .
I thought using extra-variables but it doesn’t work, args still show ‘null’
The module is elasticache and I need to try a run with hard_modify = true .
But this is not part as one of the existing variable in my playbook,
I just need to override at runtime the default value from the module (I am using Tower)
I tried the following:
elasticache:
hard_modify: true

as well as :
elasticache.hard_modify: true

but none worked and the default value is still returned.