Unfair pay deduction

This is illegal: the law is very clear that an employee's pay may not have deducations from it except with the employee consent or pursuant to a court order (like court-ordered wage garnishment). You husband could sue the employer (e.g. in small claims court) for the money, if he deems it worthwhile. However, he needs to consider whether he may in fact be liable for the employer's loss before deciding whether to sue.
As a general matter (beyond the issue of payroll deductions) an employee is not responsible for a customer's wrongdoing (e.g. not paying) unless it can be shown that the employee was at fault in some way, such as, for example, doing the work first, before getting payment, if the business's policy (known to the employee) was to get payment before doing work. IF an employee can be shown to have been at fault, the employer could sue him for reimbursement of any losses the business suffered, but would have to sue for them--it could not simply tke the money out of the paycheck. 
In this case, it depends on whether what your husband did, in letting the customer take his car and go to the bank after handing over what appeared to be a license, was careless or not: that will depend on what this employer's policies or rules where--is this how the employer handles situations like this? what is generally done in this industry when someone claims they need to go the bank--are they allowed to take the car in which the equipment was installed? etc. If you husband was careless, then the employer could sue him for the value of what was taken or lost due to his carelessness. If you think this is the case, it's better to just let matters go than sue for the withheld money, since if your husband were to sue for the money taken from his paycheck, the employer could countersue for the value of what your husband careless allowed to be taken, and so you'd net out in the same place.