If your offer is strongly over market value you will get a message in the app and can correct your offer.
If it is a typo and therefore a strong overpay, e.g. 90 instead of 9 million, we can in some exceptional cases credit the difference after you have sold the player back to the transfer market. Please send us an email with your manager ID, league and screenshots of the purchase and sale of the player. We will decide individually if the difference will be refunded.