We understand that you're feeling upset and taken advantage of, but we want to remind you of what the WEB says in Romans 12:19, "Don't seek revenge yourselves, beloved, but give place to God's wrath. For it is written, 'Vengeance belongs to me; I will repay, says the Lord.'"
Firstly, we notice that God was not explicitly mentioned in the request. It is only by the name of Jesus that we have access to God. Also, it is only in the name of Jesus that we can be saved. "for 'whoever will call on the name of the Lord will be saved.'" Romans 10:13 WEB
Also, if it were not for God, then the person would not have anything to repay since God gives everyone the ability to have wealth. Deuteronomy 8:18 WEB But you shall remember Yahweh your God, for it is he who gives you power to get wealth.
Before addressing the situation with the person, let's take a moment to pray for you both. Let us not pray for vindication, but rather for a heart that is understanding and forgiving. Also let us pray that this person comes to know Jesus Christ as their Lord and Savior.
Dear Jesus, from Matthew 6:12, we learn to pray and ask for forgiveness from others as we forgive our debtors. We pray for this situation to be resolved in a way that honors God and remember that we are to forgive as we have been forgiven. Thessalonians 5:15 "See that no one returns evil for evil to anyone, but always follow after that which is good, one toward another, and toward all." We would most of all like this person to come to know Jesus Christ and if it is your will God, then for the money owed to be returned. We pray that you work in the heart of the person to return what is owed if it is your will. We trust and believe that all things work together for good to them that love God, to them who are the called according to his purpose. Romans 8:28 WEB. We pray that this situation is resolved to the benefit of everyone involved. It is only by the name of Jesus that we have access to God. Thank you for hearing our prayers. We ask this all in Jesus' name. Amen.