If God is perfect and needs nothing, why would he demand a human sacrifice for our sins?
The short answer
Jesus' sacrifice was not merely a human one: he redeemed us as God. Adam's sin offended an infinitely good God, yet Jesus could have simply willed our redemption. He freely chose to suffer and die to show us something of his infinite love, which no payment of ours could ever match.
If Jesus' sacrifice were merely a human sacrifice, it would not mean much. But Jesus was not just a man; he redeemed us as God.
We must think outside the human box. We tend to think eating a piece of fruit is not so bad, but Adam's sin was against an infinitely good God, who is not just a bigger version of us, so it was a great offense. Even so, Jesus could simply have willed our redemption and all would have been fine; he is God.
It was the Father's will that Jesus suffer and die for us, and Jesus agreed: "No one takes it from me, but I lay it down on my own" (John 10:18). He chose to suffer and die to show us something of what his infinite love means. "Infinite" is an abstraction to us, since we experience nothing infinite, but the death of God on a cross is no abstraction. Good Friday shows how unfathomable his love is, far beyond our feeble understanding.
Forgiveness does not demand payment. We could never repay such love.