How to make and save changes to a materals .tres file?

I have a shader material. Its properties are pictured above. Id like to instantiate its "Slice Sprite UV Offset Y" property on start up.
I tried this:

var _solids_material = load("res://blocks/solids_materal.tres")

func _init():
	_solids_material.set_shader_uniform("slice_sprite_uv_offset_y", slice_sprite_uv_offset_y)
	ResourceSaver.save(_solids_material, "res://blocks/_solids_material.tres")

But the property didnt change?
Any idea what im doing wrong?
Thanks!