When making du’a to Allah for something that we are in need of, we should be firm in our request, acknowledging that we are in need of Allah’s assistance, and expecting Him to provide it. Anas ibn Malik, may Allah be pleased with him, narrated that the Prophet, peace and blessings of Allah be upon him, said:
“When one of you makes a du’a, then let him be firm and determined in his du’s, and let him not say, ‘O Allah! If You will, then please forgive me’, for there is no one who can force Allah to do anything.” (Recorded in al-Bukhari)
It is however permissible to use this approach if we are unsure is a matter is good for us. For instance we might say: “O Allah! If you know such-and-such a matter (related to this world) to be good for me, then grant it to me”, such as we do in Salatal-Istikharah and this is permissible. What is prohibited is to use this type of phrase in a matter where there is no doubt about our needs, such as Allah’s Forgiveness, or Mercy or Blessings etc.