The majority of scholars are of the view that gold may be added to silver to complete the nisaab.
The Shaafa‘is held a different view, and did not think that they can be put together, because each of them is a separate category by itself, just as wheat cannot be put together with barley and sheep cannot be put together with cattle. The same also applies to gold and silver.
Shaykh Muhammad al-Mukhtar ash-Shinqeeti (may Allah preserve him) said:
This is a matter concerning which there is a difference of opinion. For example: a man has half of the nisaab in gold, and he also has silver which, if it is added to the gold, it will reach the nisaab. Then we have this issue: can each of them be regarded as a separate category on its own, or can they be put together?
There are two scholarly views concerning this matter: some of them think that they should be put together, and others – namely the Shaafa‘is – think that they should not be put together.
The Shaafa‘is quote the basic principle as evidence. They say: Islam regards silver as one type of wealth and it regards gold as another type of wealth, and we cannot put one type together with another type, just as we cannot put camels together with cattle, or sheep with camels. Because they are of different types, we should look at each one on its own. This view is based on the fundamental principle, meaning that gold is one type and silver is another type. The evidence for that is the proven report in as-Saheeh from the hadeeth of ‘Ubaadah ibn as-Saamit (may Allah be pleased with him), who said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “Gold for gold, silver for silver, wheat for wheat, dates for date, barley for barley, salt for salt, like for like, hand to hand, same for same, and whoever gives more or asks for more has engaged in riba (usury). But if these commodities differ, then sell as you like, as long as it is hand to hand.” According to consensus it is permissible to sell gold for silver of varying amounts, which indicates that they are two different types. This is according to consensus with regard to exchanging gold for silver. If the Islamic view is that they are two different types in the case of exchange, the same is also applicable in the case of zakaah.
This is the view that has the stronger evidence and is more likely to be correct, in sha Allah. Each of them has its own nisaab, and it is essential for silver to reach its own nisaab and gold to reach its own nisaab, contrary to the view of the majority (may Allah have mercy on them), who said that each of them may be put together with the other.
End quote from Sharh az-Zaad
For more information, please see the answer to question no. 144734
With regard to cash, the correct view is that it may be put together with gold or silver to complete the nisaab. If the owner of the wealth has cash, but it does not reach the nisaab of either of them, gold or silver, but he has one of these two types of wealth, gold or silver, enough to complete the nisaab if it is put together with this cash, then he must put the cash together with whatever will complete the nisaab of gold or silver.
It says in a fatwa pf the Standing Committee (13/399):
Thirdly: obligation of zakaah on cash, if its value reaches the lower of the two thresholds, gold or silver, or it will complete the nisaab if put together with athmaan and trade goods, if they are owned by one for whom zakaah is obligatory [?] end quote.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked:
If he has gold and some cash, neither of which reaches the nisaab on its own, can he add the gold to the cash?
He relpied: If he has half of the nisaab in cash, and half of the nisaab of gold, then he may put them together.
End quote from ash-Sharh al-Kaafi
And Allah knows best.