In exchange for not sending ammunition to Ukraine, what should China deliver to Pakistan?
It's a slippery slope once you start dealing like that. It's essentially allowing someone to successfully hedge a strategy of blackmail against you where you give them something and they simply don't do anything. They will get used to that and learn that when dealing with you, they don't have to offer anything back; they just have to create some trouble and then not follow through to get free things from you. Only make exchanges where both sides give and take. The benign response to this type of offer is to reject it. The more aggressive reaction is to cause a larger problem for the blackmailer so that he rescinds his actions in exchange for you rescinding yours. But there are two further points.
1. China and Pakistan have a very amicable relationship so this blackmail exchange is not on the table. If there are problems, the countries can talk and work it out reasonably.
2. Pakistani ammunition is not going to make a difference to Ukraine or Russia. See all the US puppets bleeding themselves out to give to Ukraine? Pakistan's little gift is a drop in an ocean. If Pakistan has its own reasons to do this, whether it be revenge for Russia supplying India or to gain something from the West, China doesn't necessarily need to stop it.