is cheating in a tennis match (letting them tell you that the score was really something else when you know that you were losing but they say that it’s tied but you just agree with them that its tied) is that mortal sin?
Lying is a sin. You knew it was wrong. You certainly comitted it with some thought. That is two of the three requirements for a mortal sin.
The question then is this a grave matter. That might depend on what is at stake. If it is a pick-up game among friends, it is probably not a grave matter.
If it is a competative tournament with prize money or prestige at stake, then that is certainly more serious. For example, a high school tennis tournament may lead to state finals. The winner, and even those who place at the state level, may qualify for athletic scholorships. In this case, I believe it would be a grave matter as you may be potentially both lying which may cause a better player to not advance in the standings potentially denying them a scholoship.
Let’s say the above example is the case. What should you do, in addition to confession? The remedy may be harsh. You should probably contact theose in charge of the program and explain to them the situation, that you conscience will not allow you to advance and that the person whom you “beat” should advance. Basically, give up the ill-gotten gains. Just like a theif who must return what he stole.
As hard as this sounds, there is absolutely nothing worse that a guilty conscience. If you can do this, the respect you will have for yourself, the respect you will earn will be amazing.
How about bribing a church member to be doing something that I am suppose to do is this sin?