How do you validate the claimed expenses

[Saman Jafari]

Hey all, 

I am having a  hard time with my property managers to validate the expenses they claim. For example, one of them changed tge AC for around $1500 without getting approval and this month I get nothing from two properties because this expenses and some others that are not clear for me what those are. 
I am wondering what is the best practice for being able to grow ? 
I know that some of these can be addressed in a contract for getting approval and etc. Or I was thinking to have an accountant to check their expenses end of month and validate the expenses they did ( not sure anyone else here  takes this approach) but curious to know what your best practice is that works