The high-level meaning of a transaction is to send all the contents of an address. So if you only want to send partial payment, then you need two transactions - one to the foreign destination address and one back to yourself for the remaining funds. The analogy is closer to law and property title, rather than the mathematical/accounting actions of addition or subtraction.

In fact addresses don't really exist, other than as named state in the chain of title. It is the prior, unspent transaction output (utxo) that is unlocked during a transfer, and not some abstract bitcoin address.

Other blockchains, can and do handle these things differently.