The short answer
There is no difference in value between RMB, Chinese Yuan and CNY. They all refer to the currency of mainland China. RMB is the currency name, Yuan is the unit, and CNY is the three-letter code used internationally.
Where each term comes from
RMB is an abbreviation of renminbi, the currency name. Yuan is the unit, used in the way you would say pound or euro. CNY is the ISO currency code, used in banking and software because codes are unambiguous across languages, where names are not.
A fourth term, CNH, describes the offshore-traded renminbi. You may encounter it in financial news. It is not a different currency and it does not change what a supplier invoice in CNY is worth.
When the distinction matters — and when it does not
The distinction matters in financial trading, in accounting systems, and in environments where every field must be machine-readable. It does not matter for a supplier payment, a family transfer, or a conversion from Ghana Cedis into Chinese currency through ELITE RMB.
So if your supplier writes RMB on one line of an invoice and CNY on another, treat them as the same requirement. The figures should match, and if they do not, that discrepancy is worth querying with your supplier before you pay.
What to check instead
Rather than worrying about the naming, check the two things that actually determine whether a payment succeeds. First, is the amount agreed and confirmed in writing? Second, do you have the correct recipient account details — the account type, the account identifier, and the account holder name exactly as it appears on the receiving account.
Those two checks prevent more failed or delayed payments than any terminology confusion.