ru is optionally substituted for the n of nṝn before p,
The अ in प is for the sake of pronunciation only. Thus नॄँः पाहि or नॄंः पाहि; नॄँ प्रीणीहि or नॄँः प्रीणीहि ॥
Why do we say 'before प्'? Observe नॄन् भोजयति ॥ Some read the anuvritti of उभयथा into this sutra, so that it is an optional one. Thus we have नॄन् पाहि also. The nominative case in नॄन् has the force of Genitive.,