Question Summary:
Is it permissible to settle a bank loan by money received via lodging a disservice claim against the same bank?

Question Detail:

 
This is the situation as follows:
 
My sister has taken out an interest base loan. She knows this is wrong, and is a major sin. That's not what this is about.
 
She is trying hard to pay it off as soon as possible. She makes extra payments whenever she can afford it. Now recently she made a one off payment online, and the bank did not send her a receipt for this payment. This disservice gave my sister an idea. She lodged an official complaint with the bank who in the end send her a receipt, and offered her £160 as a token of their apology for the disservice, and also because of the poor customer services by other bank agents who were not treating her and her complaint accordingly. She gladly accepted it, and paid it on the loan which, is taken out or owed to the very same bank. Please bear this in mind.
Now, my sister only complained not because she was genuinely upset she did not get the receipt, but she knew this is sort of a loophole she can exploit because from the bank's point of view they would regard this as a disservice hence why they accepted and worked her complaint. So, she was playing a sort of deceptive game.
A very grey area.
So, our questions are:
 
a) Was it acceptable for her to accept the funds from the bank bearing in mind they offered, and she raised the complaint for an anterior motive.
b) She's already paid it on the loan, so is she obligated to return the same amount (£160) back to the bank even though the loan is with the same bank?
c) We’ve also consulted other scholars’ opinions on this, and if we get mixed answers, and end up doing what’s convenient which, is not paying it back is there a chance my sister will be liable for this sin on the Day of Judgement? Or will that Hadith apply where if one scholars gets a fatwa right he gets 2 rewards, and if he made a mistake he still gets 1 reward which, by extension means the people involved will not be sinful either, but forgiven since they at least consulted the relevant people of knowledge?
 
Jazak'Allah.

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.

And Allah Ta’āla Knows Best
Abdullah ibn Mohammed Aijaz
Student Darul Iftaa
Baltimore, USA
Checked and Approved by,
Mufti Ebrahim Desai.

 The legal status of following a Madh’hab by Mufti Muhammad Taqi Usmani