What is the ruling for a debtor who isn’t able to settle his debts?
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions and all those who follow his footsteps until the Last Day.
A debt is loaning ownership of something to another person on the condition that the person returns it to him without any additions. 
We would like to emphasize that it is obligatory to settle one’s debts. Allah SWT will ease make it easy for him by granting him ways to settle his debts if he sincerely set his intention to return the debt. This is as stated in a hadith narrated from Abu Hurairah RA, where the Messenger PBUH said:
مَنْ أَخَذَ أَمْوَالَ النَّاسِ يُرِيدُ أَدَاءَهَا أَدَّى اللَّهُ عَنْهُ، وَمَنْ أَخَذَ يُرِيدُ إِتْلاَفَهَا أَتْلَفَهُ اللَّهُ
“If anyone accepts other people’s belongings meaning to pay back, God will pay back for him; but if anyone accepts them meaning to squander them, God will on that account destroy his property.” 
According to the above question, the ruling of postponing or delaying the payment of one’s debt for those who are capable to settle it is prohibited. This is mentioned in a hadith narrated by Abu Hurairah RA, the Prophet PBUH said:
مَطْلُ الْغَنِيِّ ظُلْمٌ وَإِذَا أُتْبِعَ أَحَدُكُمْ عَلَى مَلِيءٍ فَلْيَتْبَعْ
“Delay in payment by a rich man is injustice, but when one of you is referred for payment to a wealthy* man he should accept the reference.” 
For those who are incapable, it is best if the creditor postpones and delay the process of settling the debt. This is based on the statement of Allah SWT:
وَإِن كَانَ ذُو عُسْرَةٍ فَنَظِرَةٌ إِلَىٰ مَيْسَرَةٍ ۚ وَأَن تَصَدَّقُوا خَيْرٌ لَّكُمْ ۖ إِن كُنتُمْ تَعْلَمُونَ
“And if someone is in hardship, then [let there be] postponement until [a time of] ease. But if you give [from your right as] charity, then it is better for you, if you only knew.” 
Imam al-Maraghi explained this verse saying: “If a debtor is in some trouble making him unable to repay the debt, then the creditor should wait or give some time for him to settle his debt. However, if one just let go some or all of the debt as a form of charity for the person, then there is a greater reward from Allah SWT for him than postponing the time for the debtor to settle the debt.” 
Here we present a hadith asking Allah to ease one to settle one’s debts:
One day the Messenger of Allah (ﷺ) entered the mosque. He saw there a man from the Ansar called Abu Umamah.
He said: What is the matter that I am seeing you sitting in the mosque when there is no time of prayer?
He said: I am entangled in cares and debts, Messenger of Allah.
He replied: Shall I not teach you words by which, when you say them, Allah will remove your care, and settle your debt?
He said: Why not, Messenger of Allah?
He said: Say in the morning and evening:
اللَّهُمَّ إِنِّي أَعُوذُ بِكَ مِنْ الْهَمِّ وَالْحَزَنِ وَأَعُوذُ بِكَ مِنْ الْعَجْزِ وَالْكَسَلِ وَأَعُوذُ بِكَ مِنْ الْجُبْنِ وَالْبُخْلِ وَأَعُوذُ بِكَ مِنْ غَلَبَةِ الدَّيْنِ وَقَهْرِ الرِّجَالِ
“O Allah, I seek refuge in Thee from care and grief, I seek refuge in Thee from incapacity and slackness, I seek refuge in Thee from cowardice and niggardliness, and I seek in Thee from being overcome by debt and being put in subjection by men.”
He said: When I did that Allah removed my care and settled my debt. 
We would like to reaffirm that debt is obligatory to be settled by everyone. However, if the debtor isn’t able to pay it back due to certain difficulties, then it is recommended for the creditor to postpone the time for the debt to be settled or let it go if he is financially capable and it doesn’t burden him. Hopefully, this explanation gives us all understanding of the shariah of Islam better. Amin.