Is it permissible to sell dollars for Egyptian pounds via an electronic intermediary, given the delay in receiving the Egyptian pounds after transferring the dollars to the intermediary's e-wallet?

The transaction mentioned is a money transfer service from your account to another account in the same currency. It is considered a pure agency and does not require immediate قبض (receipt) because it is not a currency exchange (صرافة). If it involves another currency, then it is both a currency exchange and an agency.

If it includes a currency exchange and the bank entry is delayed due to natural reasons, this does not affect the validity of the transaction. The delay in the bank entry is excused for customary periods, provided that the beneficiary does not dispose of the currency until they are able to physically receive it.