Why is it ruled that money be combined with gold for zakat, even if their sources and hawl (possession period) differ, while the principle is that each asset has its own hawl?

First: Gold, silver, currency, and trade goods are considered a single category, and they are combined to complete the nisab (definition) (minimum taxable amount), but not for the hawl (one-year period). This is what has been decided by Islamic Fiqh Academies and Fatwa (definition) Councils.

Second: The meaning of combining them for completing the nisab and not for the hawl is that each asset has its own hawl, and zakat (definition) on wealth is not obligatory before its hawl is completed, unless the muzakki (payer of zakat) wishes to pay the zakat in advance. So, if you own nisab of gold, and then you acquire an amount of money less than the nisab, then the money is subject to zakat by combining it with the gold. The hawl for the money is the date of its ownership. If gold is combined with money, it is permissible to pay the zakat from the gold, or to appraise the gold and pay the zakat in cash.