The debtor is the person who took the loan and must repay it. The creditor is the person owed the money.
1. Ahmad borrows 5,000 rupees from Khalid: Ahmad is the debtor and Khalid the creditor.
2. Ahmad has the amount due but repeatedly postpones payment without reason: this illustrates مَطْلُ الْغَنِيِّ.
3. Ahmad genuinely lacks the means to pay: delay alone does not establish deliberate withholding by a person able to pay.
The basic meaning of الْغَنِيِّ is being wealthy; in this narration, the relevant quality is ability to pay. Removing that condition changes the specific meaning.