Mahr difficulties - parent getting involved

My fiance and I are planning on doing our nikkah next month. He agreed to give me the mahr I asked for but he told me he can only give me 60% if it now and the rest later/ in installments. Ideally I want it upfront but I understand his situation and I told him as long as the rest is paid before the wedding (which’ll be in about a year) I’m okay with it and of course I will mention it in the contract and be specific about what I want.

I spoke to my dad and he is okay with that too since my fiance and I have this understanding. But my mom is a completely different story. She’s now against us getting the nikkah done because according to our culture the entire mahr is paid upfront. She keeps getting upset and telling me that she’s unhappy and doesn’t agree with this. Theres no way to reason with her and I’m getting frustrated.

I know ultimately since me and my dad are okay with it, she doesn’t have much of a say but I just don’t want her resenting me or my fiance or start being off putting towards him because of this.

It’s really getting frustrating now as we’ve been going at it for a few days and she’s not budging. I don’t know what else I can do…