Herlock Sholmes:
maybe this could work:
If an invitation is refused where both players have an established rating and the inviter has the lower rating and the rating difference is not all too big and the refuser has no good excuse (like being a pawn and playing the full number of games) and the players do not already play a game of this type or have played one short ago,
then the ratings of the players are updated as if the game was played and had ended in a draw.