Tell him you don't have the money to pay for a suit, whether your DP wanted to be usher or not. That isn't unreasonable especially when you get all of 2 seconds notice that you're expected to pay. Either your FIL coughs up the cash and doesn't expect to be paid back or he shoves the whole usher thing where the sun don't shine.
That is absolutely outrageous. Do you think you were deliberately not told until the last minute about having to stump up for the outfit? Your FIL sounds a bit childish and selfish to be honest, I think you're right to try to have little to do with him.
Also I firmly believe that if you ask someone to take part in your wedding you pay for what you want them to wear. If you want them to pay for their own clothes you leave it entirely up to them what they wear and how much they spend.
The father doesn't sound very fatherly...more childish and selfish I'd say, especially as he knows about your current sitution and your DP being short on work. Etiquette, and common courtesy, would dictate that if FIL wants your DP to wear a particular outift, he should pay. I think I'd point it out to him, he sounds quite stingy with his own cash.
That's so out of order. So what happened - has the suit been paid for? If so, who by? I know it's difficult but your DP needs to stand up for himself and his family, and refuse to pay. If that means your FIL has a strop and your DP's no longer welcome at the wedding, then so be it.
I actually think you are reading too much into it (although I haven't read your other post) Your fil maybe just isn't aware of wedding etiquette, I imagine many men aren't. He probably is just focussed on all his boys taking part in his big day and he didn't think his sons would mind paying- especially if he isn't well off.
I know if my mum got married again and she didn't have much money I certainly wouldn't complain about buying my own bridesmaids dress, I would just be delighted to make her happy by taking part.
I think it is completely different however to expect non immediate family or friends to pay.
Not that it's right but did your DP accept the 'loan'? If so it's hard to say now, 'oh i cant pay you back'. He should have calmly said 'oh i didn't realise we had to pay - in that case, i cant really afford it dad and i have a nice suit of my own to wear' (hindsight is fantastic when you aren't flustered!)
Can you offer to pay the dad back a £1 per month - or something paltry like that?