Ultimate financial responsibility for business expenses?

[Guest rslough]

What regulations, if any, restrict employers from requiring that employees bear the financial responsibility, in whole or in part, for expenses incurred for activities (such as travel on Company business) undertaken at the explicit request or direction of the employer for the employer's benefit?

Can an employer penalize an employee for declining to engage in an activity when said activity would require the expenditure of personal funds which the employer has established will not be subject to reimbursement?

Doesn't such a policy amount to defacto wage garnishment?