What is the meaning of this hadith and especially the part where it says "and do not sell for ready money something to be given later".
Abu Salid al-Khudri reported Allah's Messenger (?) as saying:
Do not sell gold for gold, except like for like, and don't increase something of it upon something; and don't sell silver unless like for like, and don't increase some thing of it upon something, and do not sell for ready money something to be given later.
(Sahih Muslim 3845)

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The Hadith that you mentioned was cited by Al-Bukhari and Muslim, may Allaah have mercy upon them, on the authority of Abu Sa‘eed Al-Khudri . One version of the Hadeeth reads: “…unless it is (exchanged) hand to hand.” Another version reads: “…except weight for weight, like for like.”

As for the meaning of the Hadeeth, the statement of the Prophet : “Do not sell gold for gold except like for like,” means: do not sell gold for gold except that the weights of the two transacted items of gold are equal. For example, if I sell you twenty grams of good gold in return for fifteen grams of gold of a lesser quality, the sale transaction is invalid. The weight of the two transacted items of gold must be equal.

As for his statement, “Do not make part of it greater than the other,” it means do not make any of the transacted amounts greater than the other (i.e., both weights must be equal), and it is a confirmation of the previous statement, “like for like.” The same meaning applies to the following statement about silver, “Do not sell silver for silver except like for like, and do not make part of it greater than the other.”

His statement, “Do not sell some of it in return for something to be paid later,” means do not sell gold that is present (in the contract session) in return for a deferred amount of gold, nor silver that is present for a deferred amount of silver, nor gold that is present for a deferred amount of silver, nor silver that is present for a deferred amount of gold. Rather, gold and silver must be exchanged hand to hand during the contract session. It is invalid that I sell you gold and give it to you on the spot in return for a deferred amount of gold or silver payable, for instance, on the following day. The exchange of the transacted commodities must be done in the same contract session. Musannaf ‘Abdur-Razzaaq reads:

‘Umar said: “Do not sell gold for gold, nor silver for silver except like for like (in weight), and do not increase any of the transacted items to be greater than the other. Do not sell that of it which is present in return for something to be paid later. If someone asks you to wait for payment until he goes inside his house, do not agree to this. I fear for you to fall into Riba (interest).”

An-Nawawi said: “The statement of the Prophet, : ‘do not sell for ready money something to be paid later’ means do not sell what is present for a deferred amount. Muslim scholars unanimously agreed on the prohibition of selling gold that is present for a deferred amount of gold or silver, and the same applies to selling wheat for wheat or for barley and it applies to (ambiguous and misleading without the added clarification “it applies to”) exchanging any two commodities that share the underlying cause for Riba.” [End of Quote]

Allah Knows best.