Using a nested IF Function and AND function on 2 sheets?

I am not sure if this is the correct type of formula so I'm looking for some help.

I attached the workbook so you can see what I am working on.

Basically what I am trying to do is as follows,

If Sheet 1 Col B Matches Sheet 2 Col A (Unit Type) I want it to put in Sheet 1 Col F one of two things:

IF Sheet 1 Col G (Market Rent) Matches Sheet 2 Col E for that Unit Type (Sheet 1 Col B or Sheet 2 Col A) then put Sheet 2 Col G
IF it does not match for that unit type then I want it to Subtract the Sheet 2 Col E from Sheet 1 Col G (Market Rent) add that to Sheet 2 Col G and return that number in Sheet 1 Col F.

Figuring out what I am talking about is probably harder that the actual formula so please let me know what clarification will help! I greatly appreciate any efforts on this!