What is the ruling on cancelling a conditional vow before the occurrence of the thing upon which the vow-taker conditioned his vow, knowing that the vow-taker swore an oath to fulfill his vow and even said, "I swore by Allah that if I did not fulfill this vow I would not enter Paradise"? I am afraid that I will not be able to fulfill the vow; should I offer expiation? Could you, please, answer my question in detail?
May Allah bless you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.

When the person takes a valid, binding vow, then it is impermissible for him to cancel it, even before the occurrence of the thing upon which he conditioned his vow. However, if the vow-taker specified a certain time frame for the fulfillment of the condition upon which he conditioned his vow and this time elapses before this condition is fulfilled, then it is not incumbent on him to deliver on his vow. This is because the thing upon which he conditioned his vow did not take place during the specified time. Moreover, the vow-taker is not obliged to deliver on his vow if he is unable to fulfill it due to a case of inability that is not likely to end. In this case, he should offer an expiation like that for breaking an oath.

Thus, you now know that it is impermissible for you to cancel your valid, binding vow after having taken it, especially if you emphasized on fulfilling it with an oath. When the thing upon which you rendered your vow conditional takes place, then you should fulfill your vow unless you are unable to fulfill it due to an inability that is not likely to end. In that case, you should offer an expiation like that for breaking an oath, as aforementioned.

Allaah Knows best.