When a client is not paying on time, no matter what excuses he gives you, at the end, you still have done work and not getting the money for it. That's a fact.
Now, it's not the first time he's delaying payments, even after being asked and asked again. You gave him a deadline last week. He pays, end of the story. You learnt a lesson for no or little money, but you'll have to make sure that this doesn't happen again. Now, he doesn't pay...
You have multiples choices to handle this situation:
- The hard way: let him go. Forget about the money. A client who doesn't pay on time is usually not worth the hassle. You should not bother and waste time and energy chasing your earnt money. If they're having financial problems, you'll face the issue again and again. They might be trying and take advantage of you, just like scammers do.
- The tough way: you stop working until you are paid. You inform them in a very formal and professional way that work will resume as soon as you have the money in your pocket. You can also send a new invoice with late fees (like: 10%). Include a schedule with increasing late fees (+15% / +25% / +50%...) for each week late. In this case, make sure it's legal in your country and know that you can upset the client, even maybe lose him.
- The soft way: you set a last barrier, you inform the client that no more extra work can be done until full due payment is received. You send a friendly reminder and keep their ship afloat by bailing out the water but you row for free no more. There is the risk of loosing more time, money and energy though.
Now, even if it's a little late, you've probably learnt by now that you should never ever deliver the full product before you get full payment. We all did mistakes like that, based on trust or lack of boundaries. Just don't let that happen again.
I've tested all three options mentioned at least once for the last 5 decades. Option #1 loses money but saves time and energy, helps protect you mental health if you know how to get away with the loss. Option #2 is the most professional and effective way. It protects your business ethics while saving you time and money as well. Helps putting the mental charge aside for a while. Option #3 is the best way to waste a lot of time and money, with bitterness and sometimes anger at the end. I did that for almost a year for a friend's client because I didn't want to drop any of them. Lost $15.000 worth of work and paid $3.500 taxes on the money I never got.
Therefore, in your case, because you want to protect your investment and not lose the client right away, my best advice would be to use option #2. You can navigate between options, pick a little of one here and there, but stick to standing your ground: no monee, no workee.