I've also tested this issue on my own OpenERP (V7.0) instance and  on runbot, and i think it's a bug.

The payslip is computed, but the new input is not taken into account.

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Edit the file hr_payroll.py (under ../addons/hr_payroll/) around line 486 and add : 'amount': input.amount,

I tried putting this code in a salary rule, but not calculated correctly!!

You need to define appropriate "Salary Rules" based on "inputs" variable and python code, for example (fo "Other input" - code BONUS):

result = contract.wage * inputs.BONUS.amount / 100

In V8 this rule works for me:

        <record id="l10n_pl_hr_payroll.hr_salary_rule_premia" model="hr.salary.rule">
            <field name="code">PREMIA</field>
            <field eval="5" name="sequence"/>
            <field eval="1" name="appears_on_payslip"/>
            <field name="condition_range">contract.wage</field>
            <field name="company_id" ref="base.main_company"/>
            <field eval="25.0" name="amount_percentage"/>
            <field name="condition_select">none</field>
            <field name="amount_percentage_base">PREMIA</field>
            <field name="amount_select">code</field>
            <field eval="1" name="active"/>
            <field name="name">Premia wg bierzącej stawki procentowej pracownika</field>
            <field name="amount_python_compute">result = contract.wage * inputs.PREMIA_PRACOWNIKA.amount / 100</field>
            <field name="category_id" ref="hr_payroll.ALW"/>
            <field name="quantity">1.0</field>
        </record>