I tried to follow your suggestion.

What i did:
I created 2 views and inherited the hr.expense.view.form.

I inserted this code in non-admin group
<data>
<xpath expr="//field[@name='total_amount_currency']" position="attributes">
<attribute name="groups">!base.group_erp_manager</attribute>
<attribute name="readonly">state in ["approved", "done", "submitted"]</attribute>
</xpath>
</data>

and this for admin group,

<data>
<xpath expr="//field[@name='total_amount_currency']" position="attributes">
<attribute name="groups">base.group_erp_manager</attribute>
</xpath>
</data>

Just like what you said. But, what only happens. The total_amount_currency field became invisible to admin group. But it really read-only in non-admin group.

hmm, i mean you can adding 2 field, not replacing the attribute. if you do replace the attribute, it will only exist 1 fields (thats not what i expected).

Oh. You really literally mean 2 fields. Well that's not what i actually want..

i think we got miscom,
i mean 1 field from model, but you defined it into 2 field view in xml