My husband recently quit his job to take another. He has been diligent in training another employee to take over his duties, has returned all of the items belonging to the company, and has tied up all of his loose ends. The employer gave my husband his blessing, and wished him well.

For the job he is leaving, he traveled quite a bit, and received "per diem" pay for some (not all) of the days that he was away from home. (The days he was at another site, he got paid for, but did not get paid extra if he wasn't on site, but still out of town.) Is the company required to pay those "per diems" on his last paycheck, or are they able to hold them until they get paid from the companies that my husband was at? It usually takes months for some of them to pay.

This is complicated. Not all law is labor law. There are exactly two states which consider expense reimbursement to be a labor law question. Neither state is Michigan. I am not saying that some type of court recourse is not possible, but I am fairly sure that there is no hard law in MI requiring the employers reimburse expenses quickly (or at all).

As long as the employee is on good terms with the former employer, waiting might be prudent.