Daily allowances paid by an insurance (sickness, accident, maternity, military service, disability, loss of earnings) are entered as third party payments: the automatic 2050 line deducts them from the payslip so the employee is not paid twice, and takes them out of the insurance bases.

Because that insurance money is free of social contributions, the deductions no longer match a normal month and the final net drifts away from what the employee normally receives.

Some employers promise the employee the exact same net as if nothing had happened. This commit adds a "(Net compensation)" variant of each daily allowance wage type to express that promise: when one of them is used, the payslip computes wage type 4900 (Net/Gross compensation) on its own, with the amount that brings the final net back to exactly what it would be without the allowance lines. The employer takes over, or gets back, the whole difference in contributions and source tax.

There is no direct formula for that amount, since the 4900 line changes its own deductions (source tax brackets, insurance ceilings, 5 cents rounding). The payslip is therefore recomputed a few times with different amounts until the net matches, each attempt running inside a database savepoint that is rolled back afterwards, so nothing of those attempts is kept. A manual 4900 input keeps priority over the automatic computation, the same way a manual 2050 input does.

The new wage types get the same accounting mapping as their normal counterparts.

Forward-Port-Of: odoo/enterprise#128243