Is repentance accepted before returning the debt to its owner, given the delay in payment?

We understand from your question that you delayed the repayment of a debt that was due without informing or agreeing with the creditor.

If you are in hardship (financially distressed): There is no sin upon you for the delay, and the creditor does not have the right to demand payment from you, as Allah (the Exalted) says: ﴿And if there is one in hardship, then grant him respite until ease.﴾

If you are affluent (financially able): - If the creditor demands the debt from you: It is obligatory for you to repay it, and you commit a sin if you do not, and you would be an oppressor, based on the saying of the Prophet (peace be upon him): "The procrastination of the rich (in paying debts) is an injustice that makes his honor and punishment permissible." - If the creditor does not demand payment from you: - Some scholars are of the view that you do not commit a sin, because the procrastination, which is injustice, only occurs after a demand. - Some are of the view that you commit a sin if the reason preventing him from demanding is shyness. - Some scholars are of the view that it is obligatory to repay an immediate debt promptly and a deferred debt upon its due date, even if the creditor does not demand it.

What is fitting for a Muslim is to promptly repay the debt when capable, out of caution against sin and in fulfillment of the promise.