I have lent a sum of money to my colleague, but she delayed paying it; therefore, I vowed to sacrifice a sheep for Allah`s sake if I got my money back. One day, she paid me that sum, but when I asked about the price of the sheep, I found that it was equal to the sum that I had lent to her. Should I fulfill my vow regardless of the sheep`s price and my living conditions, or what should I do?