OP, I've just gathered here the points from your posts which jumped out at me, re-ordered.
"DP has a lot of excellent qualities but he is extremely greedy when it comes to money."
"He has some savings but they're none of my business."
"It’s me that has said in this thread that his savings are none of my business, he hasn’t said it to me."
"He wants the money [from the sale of your flat] to make home improvements but with the view that it’ll make our property worth more. And he’s never suggested using any of his savings for this."
"I very much feel the goal is to add value on to the property rather than him actually having much interest in a conservatory."
"I'm not particularly interested in the home improvements he wants to make but I'm willing to contribute. 50:50 would be manageable."
"I very much feel the goal is to add value on to the property rather than him actually having much interest in a conservatory."
I find it very telling that he’s never suggested using any of his savings for the 'home improvements' he's proposing. You selling your flat to fund it while his savings remain untouched effectively transfers half of what you spend to him. Greedy, indeed
. And not a little bit sly.
"He’s not mean with money in general but he is greedy. He will spend but always has his eye on the next thing."
"He's not typically stingy with things like gifts/holidays but he talks a lot about what he perceives other people to have and seems quite jealous."
And he perceives your friend Becca has something he wants. That makes him jealous. And covetous. And he wants it taken away from Becca and given to him, disguised as 'home improvements' that you (nor he!) actually want.
I wonder what is more important to him - taking it away from Becca, or having it himself?
"It's become a really touchy subject for us and he keeps saying that I want to keep the flat because it reminds me of my 'wild single days' and that I'm not fully committed to the relationship until I let go of the flat."
And this is where I would read him the Riot Act. The reason you want to keep the flat are nothing of the sort - and he knows it. He's just trying to find a button to press that will make you give him your money, and to hell with your friend and her daughter. Gimme! Gimme! Gimme!
I despise manipulation.
I would throw his accusations back in his face, and question his commitment to you, that he'd rather syphon money out of you and into his pockets than see you happy knowing that Becca and her daughter are well-housed. Is what YOU want so much less important than him getting his hands on your money whilst his savings stay locked away in his sole account?
His many excellent qualities are at risk of being eclipsed by his extreme greed. If they haven't been already.