Is this the digital version of 'the cheque is in the post'? They say they have sent the money.
They could have accidentally sent it to the wrong account - they need to talk to their bank.
If your DH sent the details by email or possibly text, the invoice could have been hacked and changed by criminals. Sounds far fetched, but this is a thing now.
Or they could be fibbing because they either have no money or are just chancers. If that is the case, he needs to make his payment terms clear - amount and due date, and if that doesn't work, he needs to take formal action. If this ends up being the case, he also needs to think about his procedures - does he have written evidence that he was asked to do the job, how much it would cost, and what his payment terms are, so if he does end up taking them to court, they can't claim that the work wasn't done, or they never agreed to pay his bill. DP nearly learnt this the hard way as in his industry a lot of work is done by word of mouth and even by volunteers and one big project didn't pay for 3 months and we were thinking of going to the small claims court and realised he didn't have much concrete evidence that they were actually planning to pay him. Luckily they paid up just in time.
The polite thing is to send a letter before action - ie pay up within 14 days or else I'll take you to the small claims court. And if they don't pay, do it. It's fairly cheap and easy to do online.