I can't give you advice about business matters, of course, but I do understand human nature, and you need to be cautious about people taking advantage of you. Most people won't, but some will if they think they won't have to repay you. Do you really need them as customers?
Is it wrong to refuse to repay a debt you owe someone (whether it's an individual or a company)? Yes, of course it is. You have taken money from them, and if you keep it, in reality you have stolen it from them. The Bible is clear about this and similar matters: "Do not steal. Do not lie. Do not deceive one another" (Leviticus 19:11). I realize situations may arise that make it impossible for someone to repay, but when we can repay a debt, we have an obligation to do so.
Has it ever occurred to you that God is just as concerned about your business as you are, and more so? He is, because He is concerned about you and your family. People often think that God is only interested in our spiritual lives, but it's not true. God is concerned about every part of our lives, and He wants us to honor Him in everything we do.
Commit your life, and your business, to Jesus Christ. Then ask Him to give you wisdom as you face difficulties in your business (perhaps with advice from others), and to help you conduct it with fairness and integrity.