On Sept. 8th A company received $10,000 cash from McBryan Construction Co. as advance payment on rental equipment.

My question is would that be a debit to cash and a credit to unearned rental fees or would the credit be rental fees earned.

If they've paid you in advance of providing anything, then you haven't yet earned anything. That's what we use that unearned account for - it's a liability because the service or goods is still due and as yet unearned.