Answer
Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Sister, thank you for your question and interest to know the ruling of Islam concerning the money you earn or borrow.
Indeed, Islam pays due care to financial dealings. One of the main purposes of the Shari`ah is preserving wealth. Muslims are therefore ordained to strictly follow the rulings of Islam regarding any financial dealings.
A Muslim is ordained to pay back any money he/she owes anybody under any transaction, even if he/she was delayed in paying back this debt.
This payment should be based on the date of the transaction, not on the current date, as it is considered a debt.
In his response to your question, Dr. `Abdul-Fattah Edris, Professor of Usul Al-Fiqh, Al-Azhar University, stated:
Regarding the case mentioned in this question, the buyer has to pay back the price that was agreed on at the date of the transaction, not today. The price of any kind of goods is decided at the date of the transaction, not at a recent or a later date, even if the payment was delayed to a later time, and, in this case, it is a kind of debt.
Though the buyer has to pay the price as quickly as possible, the price must still be the same, however long the time it was delayed. As for the seller, he should receive his right, i.e. the price immediately, but he will receive a great reward from Allah Almighty for delaying the buyer who can’t pay immediately. Allah the Exalted says in the Qur’an: “And if (the debtor) is in straitness, then let there be postponement until (he is in) ease; and that you remit (it) as alms is better for you, if you knew.” (Al-Baqarah 2:280)
This is also confirmed in the Sunnah, as it was narrated that the Prophet (peace and blessings be upon him) said, “There was a trader who used to say to his men, “If you find anybody in strait, remit him, so that Allah may remit us,” therefore, Allah remitted him.” (Al-Bukhari and Muslim )
It was also narrated that the Prophet (peace and blessings be upon him) said, “Whoever respites anyone in strait or forgives him, Allah will include him under His shade, (i.e. on the day of Judgment).” (Muslim)
Given the above, you have to pay the price of the animal at the date of agreement, and not its price today.
Allah Almighty knows best.