As I saw my friend's wedding parties so "disaster", I suggest you follow your fiance's suggestion because:
1. your mom will remember this forever (if you insist giving her lower than she expected & she disagrees 分期付款)
2. The price of the flats (some top experts said Hong Kong's flats will be down at least 20% - 30% in these several years) - I'm waiting for this too! I already sold my flat in a "peak" price!
3. Your fiance is a mature-minded person (I think). You can rely upon on him.
4. Please don't make the relationship become bad among your mom & your fiance & your fiance's parents & you. (The relationship among her hubby & his family & my friend & her mom are really very very bad!!!)
At first, don't argue with your mom: try to say something soft to your mom - You promise you'll try to satisfy her requests & promise you'll do what you promised & you must help her if she has any difficulties even after marriage (some mothers may think their daughters won't be on her side after marriage - so that's why your mom said, "咁快幫住佢!"). You tell her you'll try your best to get what she requested!
Then after some time (several days or later), you ask her if 爭少少, could you pay by installment? If she disagrees, try to find your mom's friends/relatives to persuade her (of course, those friends/relatives must be on your side). If successful, congrats! If she insists, you may tell her you'll either borrow money from the bank & no money for other things & will pay debts for these several years to the bank or you'll get married later but your fiance is so nice that you're afraid you'll lose him if other girls rob him (nowadays girls are more than boys (suitable age for marriage))! See what your mom's responses to what you say!