Yes, interest may be agreed, but with a fixed ceiling. The parties may agree on an interest rate — whether for late payment or in any other case where interest is stipulated — provided the rate does not exceed seven percent (art. 227). If you agree on more, it must be reduced to seven percent, and any excess paid must be returned (art. 227). Any commission or benefit the creditor stipulates, if it plus the interest exceeds this ceiling, is treated as hidden interest and is subject to reduction (art. 227).
If you agreed no rate and he pays late, the legal default-interest rate is four percent in civil matters and five percent in commercial matters, running from the date of the judicial claim, unless an agreement or commercial custom sets another start date (art. 226).
Two important limits: interest cannot be charged on accrued interest, and in no case may the total interest the creditor collects exceed the principal (art. 232).
This is general legal information, not legal advice. For advice on your specific situation, consult a lawyer licensed in Egypt.