Legal Question in Business Law in Maryland

"Can" someone refuse to pay an invoice they received a year late? Yes. People can and do refuse to pay other people all the time. Whether the non-payment will stand up in court or not is another question which can't be answered without knowing more.

Presumably the question tries to understand whether someone has a legally defensible reason for not paying an invoice (or whether the person who wants to get paid has a good faith basis for filing a lawsuit to get paid). Those kinds of questions depend heavily on the facts of a specific case, will almost certainly require analysis of the particular contract, invoices and services performed. Basically the question can't be realistically answered without a private consultation with an attorney.

Can unreported opinions be referenced in a court proceeding? Usually no. Typically a lawsuit doesn't reference legal opinions in the Complaint or Answer at all. Instead, the pleadings are based on the cause of action per established case law without reference to prior court cases. Legal arguments with supporting cases often arise at summary judgment or in the context of a motion to dismiss or on appeal.

You're highly encouraged to sit down with an attorney to answer any questions you may have about a particular contract/debt. This online response tries to give very general legal information and isn't a substitute for legal advice on the specifics of your situation.