Permission necessarily involves authority, so you cannot give permission if you don't have the authority to do so.
However, if you wish to say that you are in agreement with the course of action, a common phrase is:
That's okay by me.
That's fine by me.
This indicates that your approval begins and ends with yourself, so that you do not appear to be speaking from a position of authority.
Note that these are colloquial phrases and may not be grammatically correct.
In a formal setting, you can simply express agreement:
I agree with your plan.
We are in agreement.