I raised an invoice for an amount of 20,400 dhs. Equivalent to 4,000 EURO but I received 21,000 dhs. As payment which is over of 600 dhs. I booked it as Debit - Bank - 21,000 & Credit - A/R - 20,400; then, how about the 600 difference? Please advise ASAP!

Do they want the 600 back?

If they do, credit a liability account for the remaining 600.

If they don't want the 600 back, credit revenue for 600

The question is why did they pay you the extra 600? I would set it up as a liability for now and look into why they sent you the extra 600.

Maybe because, it is a wire transaction, so the payee should account it as a gain.. not sure if realized/unrealized.. but in your part, it is your gain.