I borrowed money from a friend and I told him that I'll give it back, but now he doesn't want to take it. He tells me that I can have that money. What should I do?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

A creditor has the right to drop his right and forgive a debtor, since he is the owner. So, if this right is dropped, the debtor becomes exempted from liability in this worldly life and in the Hereafter. In other words, a dead person will not be held responsible for such debt.

Furthermore, such dropping of right is not only permissible but also a form of kindness and good turn. The doer will be rewarded with what is better in the Hereafter.

Allah Says (what means): {Who spend [in the cause of Allah] during ease and hardship and who restrain anger and who pardon the people - and Allah loves the doers of good.} [Quran 3: 134]

In addition, this action constitutes a likeable action as well as a good deed that brings the doer closer to His Lord, The Exalted. It is He Who urges the well-to-do to remit their debts by way of charity, if a debtor is in a hard time.
Allah Says (what means): {And if the debtor is in a hard time (has no money), then grant him time till it is easy for him to repay, but if you remit it by way of charity, that is better for you if you did but know.} [Quran 2: 280]

The Prophet said: “Whoever grants some time to a debtor so that he can repay or he (the creditor) remits his (the debtor) debt, Allah will shelter him (the first) under His Shade."

Thus, we state that you will be exempted from such a right (debt) on the Day of Judgment as long as the creditor dropped it.

Allah knows best.