Hi,
well it's only a receipt, without any recordings in the system except in the purchase document. IMO, the easiest way to get out of it would be to enter a qty. to receive of -<whatever quantity you received> in the line, and receive again. OTOH there is no real effect on this (no expected cost, no inventory movements), so it's a mistake without much of an effect. It will be righted by posting the invoice anyway.
with best regards
Jens