How can someone who stole a sum of money return it to its owner after repentance, given that he does not currently possess the amount, and is his repentance unacceptable due to his inability to repay it immediately?

Congratulations on your repentance. You should praise Allah who guided you. You must repent sincerely for Allah to accept what pertains to His right. As for the right of the owner of the money, you must seek their exoneration. You can apologize to them and try to reach an agreement with them regarding the repayment of the amount in installments, because the one in hardship should be granted respite, and it is better to give charity to them by forgiving the debt. Negative thinking is from the whispers of Satan, so you must strengthen your hope in Allah and read books on virtues and expiations for sins.