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Praise be to Allah.
If the case is as you describe in your question, and you have forgotten the person who lent you one hundred Riyaals, then what is prescribed in Islam is that you should give an equivalent amount in charity on their behalf. When you remember any of them, then pay him what you owe him, unless he accepts the fact that you gave charity on his behalf. This will clear you of debt and bring reward to you and to them.