Can I write such questions in a formal email?

基本上, 你的句子無咩大問題, 但係可以再修改少許。
If I don't have enough money, can(or may) I borrow from you?

如果在對話中, 表達「請求對方」可以用 can, 當然用may更加有禮貌啦, 因為 may解「要求批準」的意思。

http://dictionary.reference.com/browse/can%20 (在解釋7後的Usage note)
Can means only “to be able” and may means “to be permitted,” both are regularly used in seeking or granting permission: Can (or May) I borrow your tape recorder? You can (or may) use it tomorrow. Sentences using can occur chiefly in spoken English. May in this sense occurs more frequently in formal contexts: May I address the court, Your Honor? In negative constructions, can't or cannot is more common than may not: You can't have it today. I need it myself. The contraction mayn't is rare.

Borrow from someone 已經代表「問人借錢」, 不用指明 money。
http://www.fdic.gov/CONSUMERS/CONSUMER/news/cnsum07/sources.html
右手邊長方格內 (標題為: FDIC Encourages Affordable Small-Dollar Lending) 的第一段, 最後一句。
"So, why aren't they borrowing from their bankers?"

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但係你的問題表示, 你想將「借錢」這句子放在 email中, 我建議你用以下的方式去表達。
第一種情況: 只是假設噤問, 但真正要借的可能性極低
If I had any financial problems, would you support me?
雖然我現時經濟上未有任何問題, 但如果真的有, 你會否支持我嗎?

第二種情況: 都係一般情況噤問, 但係要借的可能性都唔係完全無
If I have any financial problems, would you support me?
如果我在經濟上出現問題, 你會否支持我嗎?

圖片參考：http://tw.yimg.com/i/tw/ugc/rte/smiley_1.gif

If I don't have enough money, can I borrow some from you?
Okay. 但更清淅及更英語化的講法是:
If I have insufficient money (for that dress), will you lend me some?
字眼的用法 - don't have enough money 應用 insufficient money.
can I borrow some from you 應用 will you lend me some 較好.

Is it possible that I borrow some money from you if I do not have enough for (the thing that you want to do by borrowing money for).

OR

May I borrow some money from you if I do not have enough?

* Using 'can' at the beginning of a question is not an appropriate way of asking, 'may' is a more polite way.

Hope this help!