Can we dole out back pay in installments — or at a lower rate?

Q. Long story short, we must pay an employee back pay for six weeks that he was not working for our company. It was deemed by the union through a grievance meeting that the employee will come back to work with back pay for the six weeks. Our question is this: How must we pay the back pay? Is it legal to ask to pay it in installments, or at a lower rate? We want to do right by the law, but a lump sum payment could cause hardship on the company’s finances. — Kate, Maryland