clinic/netforce_clinic/layouts/clinic_payment_form.xml

<form model="clinic.hd.case.payment">
    <field name="hd_case_id" invisible="1"/>
    <field name="bill_no"/>
    <field name="pay_amount" onchange="onchange_amount"/>
    <newline/>
    <field name="to_pay" readonly="1"/>
    <newline/>
    <group attrs='{"invisible":[["pay_amount","=",0]]}'>
        <field name="complete" help="Mark as complete"/>
    </group>
    <foot>
        <button attrs='{"invisible":[["pay_amount","=",0]]}' string="Cash" type="success" icon="ok" method="cash" confirm="Are you sure?"/>
        <button attrs='{"invisible":[["pay_amount","=",0]]}' string="Credit" type="default" icon="check" method="credit"/>
        <button attrs='{"invisible":[["pay_amount","!=",0]]}' string="OK" type="success" icon="ok" method="credit"/>
    </foot>
</form>