In the state of Utah can money be withheld from an employee's final paycheck for items not returned and for having to return a customer's payment due to this employee not doing the work he said he did?

Contract, union? What items were not returned? If it's a car it's one thing. If it's a stapler it's another.

So the employer was paid in advance, employee never did the work, money was returned to customer - ? How did anyone lose money? Sounds like the same money changed hands.

... can money be withheld from an employee's final paycheck for ... having to return a customer's payment due to this employee not doing the work he said he did?

It doesn't matter what contractual arrangement the employer may have had with a customer, or what money was received from the customer. If the employee was hired on a per-hour basis, the employee is entitled to be paid for the number of hours he or she worked, regardless of whether the employer was paid by the customer.

This is the difference between being an employee and being a sub-contactor.