ANSWER:

If the loan is not written down and the debtor denies it or takes a long time to repay it, then the lender has no one to blame but himself, because he exposed his property to loss.

And in this case , Duaa of this person will not be answered.

I read a hadith that says there are three individuals that, when they supplicate to Allah (Azz’ Wa Jal), their supplication is not answered. One of them was a man to whom another man owes money and he does not have witnesses over him. Does this mean if you let somebody borrow money without having witnesses your dua are not accepted? If so how does one make up for this?