It is prohibited to nag the borrower when you know he has nothing to pay with. Even passing before him is prohibited, because he will be embarrassed when he sees the lender and doesn’t have enough on hand to pay him. About this it says, “[If you lend money to My people, to the poor who are with you] do not act toward him like a debt carrier [nor shall you lay interest on him.]” (Shemos 22:23)

Just as the lender is prohibited to nag the borrower, it is also prohibited for the borrower to withhold the friend’s money which he has and say “Go away and come back [later for it]!” when he does have it. As it says “Do not say to your neighbor, ‘Go and come back [and tomorrow I will give]’ when you have it with you.” (Mishlei 3:28)