You need to buy the combine yourself and your father pay you for running his crops.  His acreage isn't going to vary that much but yours could.  You need to be buying most, or all, of the equipment for the operation from now on.  Decrease your dependence on your father in financial matters.  It will make things much simpler for all concerned.  If you want to expand, you expand.  If you want to purchase equipment you purchase it without having get your fathers approval.  Nothing wrong with discussing it with him but you need the responsibility for pulling the trigger.  Wean yourself off of depending on your father on financial matters.  In the long run it will be good for both of you. 

Edit.....couple of additional thoughts.  We don't know your ages.  How long will it be until you might be farming his land?  Do you have siblings?  If you do the cleaner the lines of ownership are and the perception of you getting a "good deal" will be minimized.  Have seen this create problems in families.