Your behavior constitutes unjust enrichment.

Although the credit card was bound to you when you were friends and the man voluntarily bound it to you, it is treated as a gift during this period. But continuing to use his credit card after the breakup would constitute unjust enrichment. If the man has evidence that you swiped his card, he can ask you to return it.

Unjustified enrichment refers to benefits that have no legal basis, or that have subsequently lost their legal basis and are recognized as benefits gained from causing losses to others, for which one is obliged to return.