Hello, BPD-Family!

While I have been working on my validation skills a lot and could reestablish a sweet and loving connection to my ex, I wanted to ask you about another tool, about how to set boundaries.
I know this i something that can escalate quickly. We are keeping our contact nothing but positive at the moment and no conflict has occured for months, I thought this was the right moment to ask you about this, so that I will be prepared in any situation where I gotta react and set them. I'd say I am acting like a little punching bag at the Moment because I am scared of losing him again and wanna change that
Maybe you have some examples of how you set clear boundaries with your partners or friends with BPD, in a way they could accept easily and they didn't feel hurt?
I can give you an example where I would like to set a boundary in the future, maybe you have advice on this:
My ex is struggling with money and I helped him out with 20 pounds in May - so nothing too big and something I really can wait for longer!

He has been asking many times if I was sure that I wanted to do that for him - it really wasn't a big deal for me!
He said he appreciated it and said thank you a million times. He told me he didn't know when he'll be able to give it back as he is struggling with money (as always), but he will give it back. I said it was okay, he can take his time and can pay it back within the next months, just when he has enough money. No rushing!
In the past he always gave my money back straight away whenever I lent something - I am not worried about not getting it back at all.
Just... .this time I am waiting for two months already, which is okay - I told him it was alright and that I do have enough while he is struggling. I jokingly reminded him two times within the past weeks and he was laughing about those jokes and told me, it's not forgotten and that I will get my money back, no need to worry - but he didn't say a word about when... .
What makes me feel a little bad about this situation is how he keeps going to the cinema weekly and how he is tweeting about buying expensive merchandise... .well from a friend I'd expect to give the money back before I spend so much money on stuff that is just for fun - that's just my opinion.

So he is spending a lot and lets me wait... .
Well, this really is a small thing and nothing I would blow up our new born friendship for.
As we plan seeing each other in October, I expect to get it back then.
Just as I know how sensitive he is when he is critisized - how would you handle this as an example of boundary setting?
To every other friend I'd just say, "Hey, before you get that merch, please give my 20 pounds back!" and it wouldn't be a big deal. It probably would make him feel attacked - so, can you give me advice on situations like this one - or heavier cases?

Thank you!