This depends on how you prove your income. If you prove your income with an income statement, the so-called "inflow principle" applies. This means: income is taken into account when payment is received. If the payment is received while you are receiving parental allowance, the payment will be partially offset against the parental allowance. It does not matter when you have performed the service for which the payment is.
If you prove your income with a balance sheet, the so-called "realization principle" applies. With the realization principle, income is taken into account at the time when the profit arose. For example, in the case of services, this is the time when you have performed the service and are therefore entitled to the payment. If you performed the service before receiving parental allowance, the payment is not counted towards the parental allowance. Here, it does not matter when the payment is received by you.