Contractor dispute

[Carlos Roldos]

I signed a contract with a marine contractor for a dock and a seawall. The contract included 112 cu yards of fill to backfill the wall. That's what he estimated. Contract stated that any extra fill will be billed at $75.00 cu yard.
The total contract was for 29540.00 of which 5534.00 was for limerock rip rap.
Today I received an overage invoice for 7800.00 for extra 104 yards of fill.

I understand that sometimes a job can go over the quote. But is it normal to go over 20% and not inform the customer until after the fact and then demand the funds?
The funds are coming out of a 203k rehab loan. The HUD contractor is saying I should have been notified that such overrun what imminent and a change order submitted.

Any help would be appreciated.