Is it considered cheating if I deal with a bank that does not accept banknotes with writing on them, knowing this and concealing the matter? And am I obliged to disclose the defects of old and worn banknotes when using them, even though they are legally circulated and accepted? And how can one repent if there is sin in this?

Truthfulness and clarity must be observed in transactions, and stratagems and deception must be avoided, as these are blameworthy characteristics. The Prophet, peace and blessings be upon him, said: "Whoever cheats us is not of us." Therefore, as long as the bank does not accept transactions with papers written on in ink, you are not permitted to conceal the writing and the ink marks on the banknotes. Seek forgiveness from Allah the Almighty for this, and you must desist from such actions in the future and adhere to the bank's conditions. As for old banknotes that are still in circulation and whose age does not affect their value, there is no blame upon you for using them, and you are not required to inform the person to whom you pay them.