George invited his friend Mark

[Bassim]

Have I made any mistakes?

George invited his friend Mark to his birthday party but hoped he would not come. He owned Mark some money he had borrowed a while ago and didn't intend to pay it back yet. He imagined Mark singing with others "Happy Birthday!" and then when they were alone asking, "What about the money you owed me, mate?" George would be embarrassed, his face would go crimson, and he had to invent a white lie to avoid hurting him. He knew that Mark would keep his cool, but inside would be burning with rage. He hated people who played games with him and always took his revenge. If he put off paying back his debt, George could expect a knock on his door one day, and that would mean the end of him. They would probably find his body in some ditch or in a rubbish tip - the inevitable fate of those who believed they could trick Mark.

 

[teechar]

Have I made any mistakes?

George invited his friend Mark to his birthday party but hoped he would not come. He owned Mark some money he had borrowed a while ago and didn't intend to pay it back yet. He imagined Mark singing with others "Happy Birthday!" and then when they were alone asking, "What about the money you owed me, mate?" George would be embarrassed, his face would go crimson, and he had to invent a white lie to avoid hurting him. He knew that Mark would keep his cool, but inside would be burning with rage. He hated people who played games with him and always took his revenge [what does that mean?]. If he put off paying back his debt, George could expect a knock on his door one day, and that would mean the end of him. [That's unclear. Rephrase it] They would probably find his body in some ditch or in a rubbish tip - the inevitable fate of those who believed they could trick Mark.

I've underlined where to make changes.

 

[Bassim]

The changes should be this:
1. he owed Mark some money
2. What about the money you owe me, mate?
3. his face would go crimson, and he would have to invent a white lie to avoid hurting him
4. Mark would keep his cool but inside would be burning with rage.
5. He hated people who played games with him and always punished those to tried to deceive him.
6. (I'm not sure if I can write this sentence correctly, but I'll try) If he would put off paying back his debt, George knew what he should expect- a knock on the door one day would mean his certain death.

 

[teechar]

The changes should be this:
1. he owed Mark some money
2. What about the money you owe me, mate?
3. his face would go crimson, and he would have to invent a white lie to avoid hurting him
4. Mark would keep his cool but inside would be burning with rage.
5. He hated people who played games with him and always punished those to tried to deceive him.

OK.

6. (I'm not sure if I can write this sentence correctly, but I'll try) If he would put off paying back his debt, George knew what he should expect- a knock on the door one day would mean his certain death.

Try:
George knew what to expect if he would (further) put off repaying Mark - a knock on his door one day, and that would mean the end of him.