I play a solo league and accidentally came across a very unusual, and unexpected situation today when Man Utd were playing Liverpool at Old Trafford - had to happen during this match didnt it!

My solo rules state that
1. If a defending player touches either an attacking player or ball in the shooting area (but outside the penalty area), the attacking team get a direct free kick. The defending side can either move 3 attackers or build a 4 man wall.
2. If a defending player touches either an attacking player or ball in the penalty area, the attacking team get a penalty.
So, during the game, Man Utd get the break and end up with a player in the shooting area with the ball dead ahead with only the keeper to beat. Chances are Man Utd will score as Liverpool do not have any defenders nearby.
Liverpool have a defending move but cannot block between attacking player and ball as not enough space. The defending player slides in and bashes the ball clear off the pitch. Liverpool claim it was accidental but Man Utd appeal that it was deliberate, plus it was a clear goal scoring opportunity.
The referee consults the rules and his only choice is to award a direct free kick. Man Utd are not happy and it is made even worse when Liverpool build a 4 man wall and the subsequent free kick is blocked.
In all my matches i think this is the first time this has happened. Fouls are usually because the defending player clips an attacking player, or knocks the ball when it isnt such a clear scoring opportunity.
I think there is a gap in my rules, as anytime there is a clear chance to score, a defender 'could' just intercept the ball and then have a chance to defend the free kick. I am wondering whether to introduce a red card, but how would you judge between a 'deliberate' foul or a player accidentally touching the ball. Plus, if I introduce red cards (which I do not use currently), would I need a red card for fouls in the penalty area too?
Has anyone else come across this scenario and what rules do you use?