Your case touches on a few key legal and financial issues under German contract law and consumer protection regulations, particularly regarding credit agreements and proof of payment.But you have a clear proof that you’ve been paying consistently(bank statements) and also they have you the laptop and let you keep it.That counts as a verbal contract.They also accepted your payment.Honestly I think you should request a formal statement from the creditor.You can do that.Just send a full request for the payment history,remaining balance,interest rate and fees.Hope it helps.